Photosensitive adhesive composition

ABSTRACT

The invention relates to a photosensitive adhesive composition of the polymerisable resin type, the hardening of which occurs by means of polymerisation and/or reticulation, characterised in comprising:—initiating means for at least one chain polymerisation reaction to guarantee the hardening of said composition and a sufficient quantity of at least one bifunctional monomer, comprising a photolabile centre with at least one photolabile entity and at least two polymerisable units, connected by covalent skeletons to said photolabile centre and located away from the cleavage sites of said photolabile centre, such that said composition loses the integrity and adhesivity thereof under the influence of a reticulating radiation causing the cleavage of the photolabile sites. The composition is particularly of application in dentistry.

The present invention pertains to a photosensitive adhesive composition.It also concerns a method for preparing certain bifunctional monomersthat are constituents of this composition, and some of these specialmonomers.

In the area of dentistry the use of adhesives is well known, such asacrylic resins, used to ensure the fixing of elements onto teeth. Forexample, for positioning an orthodontic brace, attachments are fixed tothe enamel surface of the teeth called brackets which are intended tohold the retaining wire in place. Once correction is completed, theseattachments are removed. Removal is generally achieved bytraction/twisting using a gripping device. This is often a long andtraumatizing operation for young patients and may cause substantialdeterioration of the enamel surface that is sometimes irreversible.

Similarly, mechanical deterioration of the supporting tissue isencountered when removing a crown, facet, post or inlay.

An adhesive has therefore been sought which enables two parts bonded bythis same adhesive to be separated without any mechanical damage to thesurface of said parts. This adhesive would therefore have theparticularity of losing its adhesiveness other than through externalmechanical action. Preferably, this loss of adhesiveness should beobtained without having recourse to solvents, but rather toun-crosslinking actinic radiation of said adhesive which would be themost suitable for dental applications.

In this respect, U.S. Pat. No. 4,286,047 is also already known publishedon 25 Aug. 1981 which discloses a pressure-sensitive adhesive whoseadhesiveness can be “released on demand” by exposure to UV radiation.The composition of this adhesive firstly comprises acrylate compoundspolymerizable per se, which imparts the pressure-sensitivecharacteristic to the adhesive, and secondly cationic photoinitiatorsand monomer compounds incorporating oxirane cycles. The photo-initiatorsare able to initiate the polymerization of the oxirane cycles after UVradiation. This photoinduced polymerization deteriorates the adhesivestructure and greatly reduces adhesion to the substrate on which theadhesive composition was applied. This adhesive is preferably used inadhesive tape or film form which is the most appropriate method ofapplication for pressure-sensitive adhesives.

However, said adhesives have the major disadvantage of being mixed withtoxic organic solvents (ethyl acetate and isopropanol) which must beevaporated after applying the adhesive composition to its substrate orto the polymer support of the adhesive strip. This operation isinconceivable for dental patients.

Also, the composition disclosed in this patent is difficult to use assuch other than in tape or adhesive film form: the immense majority ofadhesive applications require a viscous or paste formulation which ismuch more practical for coating surfaces. Finally, the adhesive strengthof the composition previously disclosed does not ensure sufficientlyresistant bonding in all cases of utilization.

The purpose of the present invention is to overcome these disadvanatgesby proposing a photosensitive adhesive composition of polymerizableresin type whose hardening is obtained by polymerization and/orcrosslinking.

For this purpose, and according to the invention, this adhesivecomposition is remarkable in that it contains:

-   -   means to initiate at least one chain polymerization reaction, to        ensure hardening of said composition, and    -   a sufficient quantity of at least one bifunctional monomer        including firstly a photocleavable centre comprising at least        one photocleavable unit, and secondly at least two polymerizable        units bound by covalent skeletons to said photocleavable centre        and positioned either side of the cleavage site or sites of said        photocleavable centre, so that said hardened composition loses        its integrity and adhesiveness under the action of        uncrosslinking radiation achieving cleavage of the        photocleavable units.

In the remainder of this disclosure, the term monomer shall designateboth monomers in the strictest sense and oligomers and prepolymers.Similarly, the term polymerization shall systematically denote “chain”polymerization.

According to an essential characteristic of the adhesive composition ofthe invention, it contains bifunctional monomers whose minimum structurecan be described by the following schema:

It can be clearly seen that with the adhesive composition of theinvention it is possible to obtain a cross-linked polymer containing“padlocks” in its matrix which are the photocleavable centres.Uncrosslinking actinic radiation will open these padlocks by cleavingthe photocleavable units leading to loss of crosslinked cohesion andresulting in loss of adhesiveness, since the polymer disintegrates.

According to another characteristic of the composition of the invention,it is in liquid, viscous or paste state at ambient temperature and mayalso contain chain polymerizable co-monomers which may act as reactivediluent. Therefore, the composition may free from any danger connectedwith solvent toxicity and is in addition easy to use, its physicalproperties permitting problem-free coating of the elements to be bonded.

Finally, in a preferred variant of embodiment, the initiation means forthe chain polymerization reaction are photoinitiators able to initiatethe polymerization mechanism under the action of crosslinking radiationwhose wavelength λ1 is different to wavelength λ2 for uncrosslinkingradiation. It will therefore be understood that with a single lamp andadapted filters, the cement can be polymerized very rapidly (a fewminutes) and conversely can be uncrosslinked to separate two elementscemented together by the adhesive composition of the invention.

Other advantages and characteristics will be more readily understoodfrom the following description of several embodiments of thephotosensitive adhesive composition of the invention; firstly exampleswill be given of the families of chemical compounds contained in thebifunctional monomers of the invention with examples of synthesismethods for some of these bifunctional monomers. A description willafterwards be given of the possible applicable variants of the type ofinitiation means for the polymerization reaction. Finally, compositionspreferably intended for clinical dental use will be presented asexamples.

According to an essential characteristic of the photosensitive adhesivecomposition of the invention, it contains bifunctional monomers that arepolymerizable and photocleavable. Among all known photocleavable units,consideration will be given to two main families, the aryl-diazos andbenzyls.

A first large family of photocleavable units which can be applied is thefamily of azyl-diazos having the following general structure of formulaI:

As a general rule, since these compounds are unstable in highly acid orbasic media, care is taken that the Ri and Rj substituents do not carryhighly acid or basic groups, unless the acidity or baseness is masked byvarious interactions such as inter- and intramolecular hydrogen bondsfor example in the case of acidity or intermolecular bonds existing inthe solid state. Also, in the following description of the Ri and Rjsubstituents, the hydrocarbon groups in particular alkyl, alcoxy,alkylthio, are preferably C1-C6 groups, aryloxy and arylthio arepreferably made up of 5 to 14 atoms, preferably 5 to 6 atoms formonomers in their strictest sense, but they may be chains of larger sizeif oligomers or prepolymers are considered.

These aryl-diazo units are therefore represented by the above formula I,in which:

-   -   Ar denotes an aromatic system that is monocyclic or polycyclic,        carbocyclic or heterocyclic, including in particular atoms such        as S or N, each cycle preferably having 5 or 6 atoms;    -   X denotes an atom chosen from among: C, N, O, P, S;    -   Ri is chosen from among the hydrogen, halogen groups, a        functional group whether ionic or not (with the exception of        halogenated groups able to release strong acids, such as acyl        halides), a polymerizable functional group such as those        described hereafter. The Ri substituent may also be a radical or        a hydrocarbon chain which may be aliphatic, acyclic, saturated        or unsaturated, straight or branched, a cyclic aliphatic,        unsaturated, aromatic or heteroaromatic radical, all these        radicals or chains possibly carrying substituents of Ri type or        interrupted by a heteroatom chosen from among B, N, O, Si, P, S,        a halogen or a functional group.

The notion of radical is more appropriate for designating a monomer,whilst the notion of chain is better suited for describing a straight,branched or crosslinked polymer in the entire description of theinvention.

On the aromatic cycle of the aryl-diazo unit, the Ri substituents mayform between themselves a carbocyclic or heterocyclic cycle, whethersubstituted or not, preferably comprising no more than 6 atoms to formthe cycle; the same applies, as a general rule, to substituents, inparticular those defined similarly to Ri which may be located on anyradical or hydrocarbon chain mentioned in the entire description of theinvention.

Ri may, in particular, be one or more groups of the type: alkyl—straightor branched, saturated or unsaturated, optionally substituted,aryl—aromatic or heteroaromatic, substituted or unsubstituted, alcoxysuch as methoxy for example or ethoxy, aryloxy, alkylthio, arylthio,benzyl, halogeno, hydroxy, hydroxyalkyl, thiol, alkyloxycarbonyl,aryloxycarbonyl, cyano, carbonyl, formyl, amino, carboxylic and sulfonicester, carboxylic sulfonic and phosphoric amide, carboxylic sulfonic andphosphoric acid, sulfonate, phosphonate, —OCONR′R″ group, —OCO₂R′,—OSO₂R′, —OPOOR′OR″, —R′NHCOOR″, —R′OCO₂R″, —NR′R″ (where R′ and R″represent an alkyl group, a carbocyclic or heterocyclic group,aliphatic, unsaturated, a (hetero-)aromatic group, all substituted orunsubstituted), imine substituted or unsubstituted, nitro, —N═N—R′,-Rp-Si-(ORq)₃ group (in which Rp is a hydrocarbon chain, preferably astraight alkyl chain having at least 3 C atoms, and Rq denotes ahydrogen atom, a hydroxy group, an alcoxy C1-C6 chain, or —(Si(ORq))group), a vinyl group, an acrylic group, an alcoxycarbonyl group or anaryltriazene group, among others.

If the aryl group of an aryl-diazo subunit is monocyclic, thesubstituent Ri donor groups of the aromatic cycle are preferably at paraor ortho of the diazo group and the Ri attractor groups preferably atmeta of this group.

-   -   Rj designates one or more substituents, according to the valency        of the atom designated by X. Generally, the different Rj are the        same or different and may designate an alkyl chain straight or        branched, aliphatic or unsaturated, acyclic or cyclic,        preferably C1-C6, optionally substituted by substituents meeting        the definition of Ri for example optionally interrupted by a        heteroatom such as N, O, Si, P, B for example, an aromatic or        hetoraromatic group including for example in a preferred chain        of 5 or 6 atoms at least one nitrogen or sulfur monocyclic or        polycyclic atom, an alcoxy chain, aryloxy chain, a benzyl group,        the Rj substituents may also form a cycle preferably having 5 to        6 atoms.

In addition, Rj may evidently be the residue of a chain when thebifunctional monomer compound is of polymer or oligomer size. Finally,Rj may advantageously designate a heteroatom such as preferably O, N ouP, structurally arranged with X in relation to the possibilities offeredby the valency of X.

If X is an atom of carbon C or oxygen 0 (arylazoalkyl and aryldiazoethercompounds respectively), Rj also designates an alkylthio, arylthiochain, a cyclohexyl, naphthyl group, a hydroxyethyl, cyanoethyl,acryloxyethyl group, alkyl (C1-C6)glycidyl ether or alkyl(C1-C6)vinylether, a cyclohexyl epoxy, advantageously in the case of aryldiazoalkylsan attractor group such as a cyano, nitro group, carboxylic, sulfonicand phosphoric acid, carboxylic and sulfonic ester, phosphonate, amide,carbonyl.

If X is an atom of phosphorus P, Rj also advantageously jointlydesignates the groups or atoms such that they create a photosensitiveunit of Ar—N—N—PO(OR′)(OR″)arylazophos-phonate type, in which R′ and R″are defined as previously in Ri, in particular R′ and R″ mayindependently designate an alkyl chain whether straight or branched,substituted or not, saturated or not, acyclic or cyclic, carbocyclic orheterocyclic, a (hetero)-aromatic radical, more particularly ahydroxyethyl chain, 1,4- or 1,3-dimethylcyclohexyl,1,4-dimethylparaphenyl, a methyl, ethyl, propyl, isopropyl,hydroxyethyl, cyanoethyl, acryloxyethyl group, an ether of alkyl(C1-C6)glycidyl or alkyl (C1-C6)vinyl, a cyclohexyl epoxy.

Examples of such compounds can be found for example in the article “Newarylazophosphonate-containing Polyurethanes and Polyesters for LaserAblation Structuring”, Macromol. Mater. Eng., 2002, 287, 671-683.

If X is an atom of sulfur S, Rj also advantageously and jointlydesignates the groups or atoms such as to create a photosensitive unitof AR-N═N—SO(OR′)(OR″)arylazosulfonate type, R′ and R″ being defined aspreviously. Rj may also jointly designate the groups or atoms so as tocreate a photosensitive unit of AR-N═N—SO₂R′ arylazosulfone type, orfurther an Ar—N═N—S—R′ arylazosulfide unit.

Finally, if X is an atom of nitrogen N (aryltriazene, arylpentazadiene,arylhexazadiene compounds) consideration is given to the followingformula II:

in which Ri is defined as previously and the R1 and R2 substituentscorrespond to the definition of Rj.

R1 and R2 may advantageously designate, independently from one another,each of the hydroxyethyl, cyanoethyl, aminoethyl, acryloxyethyl orhalogenoethyl groups.

R1 and R2 designate the residues of one or two organic compounds,optionally polymers, of which one initially carries at least one primaryor secondary amino group at its ends, and independently represent ahydrogen atom (but not the two simultaneously), a functional donorgroup, an alkyl chain straight or branched, aliphatic, unsaturated,acyclic, cyclic, preferably C1-C6, optionally interrupted by aheteroatom such as N, O, Si, P, B for example, a (hetero)-aromaticgroup, all these groups or radicals may be substituted by variousfunctional groups such as Ri for example.

The same indications as those given above also apply to the notion of afunctional group, with the addition of those concerning the synthesis ofthe triazene unit. The synthesis schematic of these units is based on(mono/bi)-coupling between a diazonium salt of an aromatic amine andanother amine defined by the formulas NHR₁R₂ or NH₂R₁ and, in the caseof monocoupling, is:

The R₁ and R₂ substituents are preferably such as the NHR₁R₂ or NH₂R₁amine exists in this form or other form in which it is stabilized ormade reactive, such as hydrates, ammonium chlorides for example.Preferably R₁ and R₂ do not designate halogens.

Similar to Ri, R₁ and R₂ may represent the necessary atoms to complete acycle. In the composition of the invention use may be made in particylarof aryl-triazene compounds or derivatives in which R₁ and R₂ are forexample a —N═N—R′ group, —NR′-N═NR″ group, OR group, NR′R″ group (R′ andR″ have the meanings previously given), an alkyl group such as methyl,ethyl, propyl, isopropyl, butyl, tert-butyl, an alcoxy group substitutedor unsubstituted, a benzyl group, a (hetero)-aromatic group, allsubstituted or not by substituents of Ri type, a hydroxyethyl,cyanoethyl, aminoethyl, acryloxyethyl or halogenoethyl group.

Persons skilled in the art will find examples of aryl-triazene compoundsapplicable to the adhesive composition of the invention in thepublications by Oskar Nuyken and/or his team.

Finally, the photocleaving site of the aryl-diazos is generally the—N═N—X— chain of which at least one of the bonds is broken under □2actinic radiation; in the bifunctional monomers corresponding to theinvention, the chemical skeletons binding said unit to the polymerizableunits are therefore reasonably arranged either side of the —N═N—X chain.

A second large family of photocleavable units applicable to the adhesivecomposition of the invention is the benzyl family, having the followinggeneral structure of formula III:

Generally, as for the aryl-diazos, the alkyl, alcoxy, alkylthiosubstituents are preferably C1-C6, aryloxy and alkylthio preferably have5 to 14, further preferably 5 to 6 atoms but they may be chains oflarger size. Also the Rk, Rl, Rm substituents described below mayevidently be the residue of a chain when the bifunctional monomercompound is of oligomer polymer size.

These benzyl units, which have better stability in acid medium than thearyl-diazos, are therefore represented by formula III above, in which:

-   -   Ar designates an aromatic or heteroaromatic radical (including        an atom such as N or S for example), monocyclic or polycyclic,        carrying at least one Rk substituent different to the benzyl        substituent explicitly present (nitro for example if        nitrobenzyls are considered),    -   Rk is generally one or more substituents of the aromatic cycle,        and designates an auxochromic or bathochromic substituent        possibly chosen from among the following examples: hydrogen,        halogen, alkyl chain, aliphatic, acyclic saturated or        unsaturated, straight or branched, a cyclic radical whether        aliphatic, unsaturated, aromatic or heteroaromatic (these chains        and radicals may be substituted, interrupted or terminated by a        heteratom such as B, N, O, Si, P, S or a halogen), a nitro,        cyano, alcoxy, aryloxy, alkylthio, arylthio, benzyl, arylalkyl,        hydroxy, thiol, alkyloxycarbonyl, aryloxycarbonyl, carbonyl,        formyl, amino group, carboxylic and sulfonic ester, carboxylic        sulfonic and phosphoric amide, carboxylic sulfonic and        phosphoric acid, sulfonate, phosphonate, a —OCONR′R″, —OCO₂R′,        —OSO₂R′, —OPOOR′OR″, —R′NHCOOR″, R′OCO₂R″, NR′R″ group (R′ and        R″ are an alkyl, aryl group, a carbocyclic or heterocarbocylic        group), imine substituted or unsubstituted, diazo —N═N—R′, a        -Rp-Si-(ORq)₃ group (Rp and Rq as defined for the aryl-diazos),        ether of alkyl(C1-C6)glycidyl or alkyl(C1-C6)vinyl, cyclohexyl        epoxy.

R₁₁/R₁₂/R₁₃ designate a hydrogen, an alkyl, alcenyl, alcynyl, alkylarylchain, substituted or unsubstituted, preferably C1-C6, a carbocyclic orheterocyclic chain, saturated or unsaturated, aromatic orheteroaromatic, substituted or not, preferably having 5 to 6 atoms, analcoxy, aryloxy, alkylthio, arylthio chain, an alkyloxycarbonyl group,—NR′COR″ group, —OCOR′ group, —OCOOR′ group, —OCONR′R″ group, NR′COOR″group, —OPOR′R″R′″ group, —OSO₂R′ group, —OPOOR′OR″ group, —NR′R″ group,—COOR′ group, —CONR′R″, SOOR′, —COR′ group (R′, R″, R′″ have thepreviously indicated denotations), an imine group substituted or not,hydroxy, thiol, a carboxylic acid or derivative, a halogen, nitrile, analkyl(C1-C6)glycidyl or alkyl(C1-C6)vinyl ether, cyclohexyl epoxy,-Rp-Si(ORq)₃ group (Rp and Rq as defined previously).

Persons skilled in the art will for example find examples of compoundsmeeting the definition of formula III in the publication “Photosensitiveprotecting groups” Israel J. of Chem., 1974, 12(1-2), pp. 103-113.

Generally, the photocleaving site of benzyls is the benzyl carbonexplicitly present in formula III of which at least one of the bondswith one of the R₁₁/R₁₂/R₁₃ groups is ruptured under λ2 actinicradiation; in the corresponding bifunctional monomers of the invention,the chemical skeletons binding said unit to the polymerizable units aretherefore reasonably arranged either side of the benzyl carbon underconsideration.

One category of benzyl units applicable to the invention which isparticularly efficient and does not comprise any nitro functionexplicitly at the ortho position of the benzyl function, are the benzylunits in which Ar is at least bisubstituted at the meta of the benzylfunction by an alcoxy or aryloxy chain and in which two of theR₁₁/R₁₂/R₁₃ substituents are a hydrogen atom or an alkyl group,preferably C1-C4, preferably at least one of the two substituents beingan alkyl group, while the last substituent is a chain starting with aheteroatom such as O or N for example or a functional group of ester orcarbamate type.

Also, the best preferred forms of these benzyl units are the2-nitrobenzyl derivatives, of which some are given in U.S. Pat. No.5,600,035 and U.S. Pat. No. 6,100,008, which can be given by thefollowing general formulas:

-   -   Formula 1:    -    in which:        -   AR is as previously defined;        -   R_(m1)/R_(m2) are defined as R₁₁/R₁₂/R₁₃.    -   Formula 2:    -    in which:        -   Ar is defined as previously,        -   R_(m4) is defined as R₁₁/R₁₂/R₁₃,        -   R_(m3) is defined as a hydrogen, an alkyl, alcenyl, alcynyl,            alkylaryl chain, substituted or not, interrupted by a            heteroatom such as N, O, P, Si, S, preferably C1-C6, a            carbocyclic or heterocyclic chain, (un)saturated,            (hetero)-aromatic, substituted or not, preferably 5 to 6, an            alkyloxycarbonyl group, POR′R″R′″ group, —SO₂R′ group,            —POOR′OR″ group, —COOR′ group, —CON′R′″, a COR′ group (R′,            R″ and R′″ having the denotations given previously for R′            and R″), alkyl(C1-C6)glycidyl or alkyl(C1-C6)vinyl ether,            cyclohexyl epoxy, (Rp-Si(ORq)₃ group (Rp and Rq as            previously defined).    -   Formula 3:    -    in which:        -   Ar is defined as previously,        -   R_(m5) is defined as R₁₁/R₁₂/R₁₃,        -   R_(m6)/R_(m7) are defined as a hydrogen, an alkyl, alcenyl,            alcynyl, alkylaryl chain, substituted or not, interrupted by            a heteroatom such as N, O, P, Si, S, preferably C1-C6, a            carbocyclic or heterocyclic chain, (un)saturated,            (hetero)-aromatic, substituted or not, preferably having 5            to 6 atoms, an alkyloxycarbonyl group, —R′COR″ group,            R′COOR″ group, R′R″, —COOR′ group, —CONR′R″, a COR′ group            (R′ and R″ having the denotations previously given), a            hydroxy group, alkyl(C1-C6)glycidyl ether or            alkyl(C1-C6)vinyl ether, cyclohexyl epoxy, -Rp-Si(ORq)₃            group (Rp and Rq as defined previously).    -   Formula 4:    -    in which:        -   Ar is defined as previously,        -   R_(m8)/R_(m9)/R_(m10) are defined as R₁₁/R₁₂/R₁₃.

Among all the photocleavable units presented above, particularpreference is given to arylazophosphonates, arylazosulfonates,arylazosulfides, aryltriazenes and 2-nitrobenzyls for their provencleaving efficacy, the involved wavelengths, their stability in theadhesive composition of the invention and more particularly, for the2-nitrobenzyls, their commercial availability and their better stabilityin acid medium, which may prove to be essential in some consideredapplications.

According to an essential characteristic of the invention, thebifunctional monomers used contain units that are polymerizable throughchain polymerization reaction. This reaction may be conducted by threeprocesses well known to persons skilled in the art: radical, cationic oranionic.

In a first variant the polymerization process is radical. Thepolymerizable units selected for radical polymerization are vinyl units.These are described as follows:

in which R₃, R₄ and R₅ are substituents able to activate together thedouble vinyl bond vis-à-vis radical addition chain reactions. At leastone of these substituents is a hydrocarbon chain, optionally interruptedby a functional group such as Ri or a heteroatom such as N, O, Si, P, Ssubstituted or not by groups such as Ri, linked to at least onephotocleavable unit so as to meet the structural criteria of thebifunctional monomers of the invention. More generally, R₃ or R₄ or R₅may represent a hydrogen atom, a halogen atom, a functional group, analkyl, alcoxy, chain saturated or unsaturated, substituted or not,preferably C1-C6, or an aryl, aryoxy group substituted or not preferablyhaving 5 to 6 atoms, an attractor functional group such as a carbonylgroup, a carbonyloxyalkyl or carbonyloxyaryl group, an amide group,cyano group, optionally a sulfonic carboxylic acid group, and theirsalts, with optional appropriate restrictions or exclusions to preservecompatibility with the photocleavable units, or further analcoxycarbonyl group.

To obtain good reactivity in respect of radical polymerization, personsskilled in the art may, without limiting the invention, select at leastone of the substituents from among the cited attractor groups to create(meth-)acrylic units for example.

As non-restrictive examples, below are given some structures whichrequire structural modification for their linking to a photocleavableunit using one of the methods explained below or using techniques knownto persons skilled in the art:

In one alternative embodiment of the composition of the invention,electron acceptor vinyl units may coexist in the composition that areable to create a charge transfer complex with at least one othercomplementary partner, this other partner itself, in a certain number ofcases, possibly being a vinyl unit that will be an electron donor. Thecharge transfer complex is then able to initiate the radicalpolymerization reaction under radiation of wavelength λ1. In particular,it is possible to choose an electron donor unit from among the followingexamples: styrene, cyclohexane oxide, vinyl acetate, vinyl ether, phenylglycidyl ether, exomethylene dioxolane such as4-methylene-2-phenyl-1,3-dioxolane, alkyl methacrylate, vinylpyrrolidone, and an electron acceptor unit among the following examples:maleic anhydride, acrylonitrile, diethyl fumarate, fumaronitrile,maleimides.

In the second variant of polymerizable units applicable to thecomposition of the invention, the polymerization process is cationic. Acationically polymerizable unit is a unit which polymerizes orcrosslinks in the presence of an acid or cation.

Such units may be chosen from among the following families: epoxide (oroxirane), oxetane, oxolane, cyclic acetal, cyclic lactone, thiirane,thietane, vinyl ether, cyclic ether, cyclic thioether, spiroorthoester,spiroorthocarbonate, aziridine, siloxane, styrenes.

With a view to photoinduced polymerization in particular (cf.description of initiating means hereafter), preference is given tooxirane units whose structure is defined by following formula V:

in which R₆, R₇, R₈, R₉ are the same or different, at least one of thesubstituents R₆, R₇, R₈, R₉ being a hydrocarbon chain, and generallyrepresent an atom of hydrogen, halogen, an alkyl, alcoxy, alkylthiochain, straight or branched, saturated or unsaturated, acyclic orcyclic, preferably C1-C6, optionally substituted, optionally interruptedby a heteroatom, an aromatic or heteroaromatic aryl group, an aryloxy orarylthio group preferably having 5 to 6 atoms, a benzyl group, an iminegroup, NR′R″ amino NR′R″, SiR′R″R′″, alkyl(C1-C6)oxycarbonyl,aryl(C1-C6)oxycarbonyl, amide, carboxylic and sulfonic ester, sulfonate,phosphonate, a carbonyl, cyano group, —OCONR′R″ group, —OCO₂R′, —OSO₂R′,—OPOOR′OR″, —R′NHCOOR″, R′OCO₂R″ in which R, R′, R″ represent an alkylgroup (preferably C1-C6) substituted or not, aryl (preferably with 5 to6 atoms), a carbocyclic or heterocyclic group, aliphatic, unsaturated oraromatic, substituted or unsubstituted.

Advantageously, for reasons of steric hindrance, two of the substituentsR6, R7, R8, R9 are a hydrogen atom.

Typical derivatives are for example the derivatives of diglyceride etheror of 3,4-epoxycyclohexyl.

A further particularly preferred family of units, with a view tophoto-induced cationic polymerization, is the family of units of vinylether type whose structure is defined by formula VI:

in which:

-   -   R10 and R11 are the same or different and designate a hydrogen        atom or advantageously a straight or branched C1-C6 alkyl chain,        substituted or not, saturated or unsaturated, acyclic or cyclic,        optionally interrupted by a heteroatom such as O, N, Si, P for        example, an aromatic or heteroaromatic aryl group (preferably        having 5 to 6 atoms), an alcoxy chain (preferably C1-C6), an        akylthio chain (preferably C1-C6), arylthio (preferably having 5        to 6 atoms);    -   R12 advantageously designates a straight or branched C1-C6 alkyl        group, substituted or not, saturated or unsaturated, acyclic or        cyclic, optionally interrupted by a heteroatom such as O, N, S,        Si, P for example, an aromatic or heteroaromatic aryl group        (preferably having 5 to 6 atoms).

Finally, among other examples of ethylene functions that arecationically polymerizable, a choice may be made from among styrenederivatives in particular such as styrene, para-alkoxystyrenederivatives, para-chloromethylstyrene, vinyl toluene, α-methylstyrene.

In a third variant, polymerization is anionic. Among the anionicallypolymerizable units, units of vinyl type may be chosen (according toformula IV) but which carry electro-attractor substituents such ascarboxylic ester in particular (methacrylic derivatives for example), orcyano, or which carry substituents permitting strong positivepolarization of the carbon at β of the double bond when approaching thenucleophilic species, such as is the case for example with units ofbutadiene, styrene, vinylpyridine type.

Heteroaromatic cycles may also be chosen such as those described forcationic chain polymerization, in particular oxirane, thiirane,lactones, lactames.

A method is described below for preparing bifunctional monomers groupingtogether the minimum structural criteria required for producing aphotosensitive adhesive composition of the invention. This methodcomprises several steps, namely a first step to produce a photocleavableunit or group of units of aryl-triazene or 2-nitrobenzyl type,optionally followed by a structural arrangement step of the skeleton ofthe photocleavable unit, and a last step during which thisphotocleavable unit is attached to one or more polymerizable functionsvia a chemical skeleton.

1—First Step: Synthesis of Photocleavable Units

Case 1: Fabrication of an Aryltriazene Unit.

Two principal implementation modes can be considered.

1.—First Implementation Embodiment

1°) Firstly diazotization is conducted of an arylamine (this termencompasses (hetero)-aromatic structures optionally pluri-substituted by—NH₂ amino groups without excluding any other substituent of Ri type,such as defined above) in a non-oxidizing aqueous acid medium to formthe corresponding diazonium salt. Diazotization is achieved bydissolving the arylamine in an aqueous mineral acid, such asconcentrated HCl or H₂SO₄ for example, and adding thereto, at atemperature of between −10° and 30° C., an aqueous solution of nitriteions. The solution of diazonium salt obtained is then used either assuch, or after modification using known means for the preparation of thediazonium salt of arylamine with a different anion, as is the case if adiazonium chloride is cold precipitated through the addition of anappropriate quantity of tetra-hydroborofluoric acid (HBF₄). In similarmanner, salts of hexafluorophosphate can be prepared (anion PF6⁻), oftetra(pentafluorophenyl)-borate (anion B(C₆F₅)4⁻), or salts ofhexafluoroantimonate (anion SbF₆ ⁻).

2°) The diazonium salt, in its initial or “exchanged” form, is thendissolved in an aqueous solution, over the same temperature range aspreviously, and the pH is adjusted in an alkaline range to achievediazoic mono- or bi-coupling with at least one of the primary orsecondary amino sites of an organic compound.

For this purpose, the compound carrying amino groups is dissolved in anaqueous solution, preferably of pH 7 to 8, or in an organic solvent(alkane, THF, acetonitrile) to which is added a diazonium salt at atemperature of between −10° C. and +30° C.

At the end of the reaction, the triazene compound obtained is isolatedusing any known technique depending upon chemical structure. In abiphase medium the products may be isolated by simple filtration,otherwise the products may be subsequently purified for example byre-crystallization and column chromatography.

2.—Second Implementation Embodiment

Another preparation method consists of obtaining a diazonium salt froman arylamine able to undergo diazotization in an organic medium such asethyl ether, THF for example or dry halogenated solvents such asdichloromethane or chloroform. Diazotization is conducted at atemperature of between −50° C. and +30° C. by adding to a solution ofarylamine with a Lewis acid such as BF₃, PF₅, SbF₅ for example, asolution of organic nitrite such as tert-butyl nitrite for example. Thesalt is then extracted using conventional means, in particular bywashing and filtering.

Diazoic coupling is then performed by adding to the recovered diazoniumsalt, dissolved in an inert dispersing solvent, a solution of an organiccompound carrying at least one primary or secondary amino group, thisreaction being conducted at a temperature of between −50° C. and +50° C.in the presence of a base which is preferably sodium or potassiumcarbonate. The aryl-triazene unit obtained is isolated using knowntechniques. It is to be noted that only the diazotization methodundergoes change, diazoic coupling also possibly being conducted asindicated under I.1.2°).

Case 2: Fabrication of a 2-Nitrobenzyl Unit

The synthesis variants are numerous, two important distinctions existdepending on whether the nitration of an aromatic cycle is necessary ornot to arrive at a specific 2-nitrobenzyl structure (this depends uponsource commercial products).

1. First Implementation Embodiment: Nitration

1°) A first structure type is created by nitrating benzyl compoundssubstituted in a form able to promote nitration at least at the orthoposition of the benzyl group. Said nitration can be conducted underconditions known to persons skilled in the art, which are the use ofdilute or concentrated nitric acid, either alone or combined withconcentrated sulfuric acid, acetic anhydride or the use of a nitroniumsalt (NO₂BF₄, NO₂CF₃SO₃ for example in aprotic organic solvents), or ofnitrites (for example NaNO₂ in the presence of trifluoroacetic acid), oresters of nitric acid (ethyl nitrate for example used under alkalineconditions or with a Lewis acid), or N₂O₅ in CCl₄ in the presence ofP₂O₅ in an anhydrous medium under conditions of temperature or timesuitable for the reactivity of the aromatic cycle.

2°) A second type of structure is created by nitrating benzene compoundsof which at least one nitratable position is adjacent to a precursorfunction of a substituted methylene. Among these functions, thereducible functions (aldehyde, ketone, amide, carboxylic acid, nitrile,imine, hydrazone, oxime, epoxide . . . ) are preferred functions, thereducing of said function to be conducted under conditions in which anaromatic nitro group is preserved; therefore the concomitant use isexcluded of metals and acid (for example Zn and HCl), of catalytichydrogenation conditions, of AlH₃—AlCl₃, TiCl₃, LiALH₄, NaBH₄+NiCl₂(PPh₃), NaBH₄+CoCl₂.2. Second Implementation Embodiment: No Nitration

A third type of structure is created by modifying o-nitrotoluenederivatives. Suitable techniques known to persons skilled in the art arebromation of an o-nitrotoluene for example in the presence ofN-bromosuccinimide or the treatment in polar organic solvent (such asDMSO for example) of derivatives of o-nitrotoluene with paraformaldehydein the presence of strong bases (such as KOH, tertiobutylate potassium,Triton B, DBU, guanidines for example) to arrive at derivatives of2-(nitrophenyl)ethyl.

II—Second Step: Structural Arrangement

Generally, when the skeleton of the photocleavable unit obtained afterthe first step has functions capable of directly fixing a polymerizableunit such as defined in the invention, it is not essential to modify thechemical structure of this skeleton, and in this case this step isomitted to pass directly to step three. If not, persons skilled in theart are able to implement the necessary reactions to achieve fixing ofthis polymerizable unit during the third step of the method.

III— Third Step: Association of Polymerizable Units with PhotocleavableUnits

Case 1: Polymerizable Units of Vinyl Type

Here consideration will be given to all the vinyl units previouslydescribed, namely those that can be radically polymerized (formula IV),cationically polymerized (vinyl ethers of formula VI and styrenederivatives) or anionically polymerized.

Starting with the chemical structure surrounding the photocleavable unitor units, there are two variants for associating vinyl units withphotocleavable units. A first variant to elongate the skeleton andinclude therein one or more vinyl polymerizable functions, is a processduring which the vinyl function is created.

The other variant consists of grafting pre-existing vinyl units.

1°) First Variant: Creation of the Vinyl Function

This preferably involves creating a vinyl function of acryloyl typeillustrated below in the case of a photocleavable unit of aryl-triazenetype.

This variant for creating the acryloyl function consists of using areaction of nucleophilic substitution type on an acryloyl carbon.Different nucleophilic agents can be used, a distinction being made ifnecessary between possibilities applicable to aryl-diazos, in particulararylazophosphonates and aryltriazenes, and to benzyls in particular2-nitrobenzyls.

1°)1) The Nucleophilic Agent is an O—R Group as is the Case for:

-   -   a) attack by an alcohol or alcoholate on acryloyl halides,        preferably conducted in the presence of a base such as pyridine;    -   b) attack by an alcohol on an acryloyl anhydride, preferably        catalyzed by a base such as pyridine of        4-(N,N-dimethylamino)pyridine (DMAP);    -   c) attack by an alcohol on an acrylic acid.    -   for aryl-diazos: esterification reactions abound in the        literature and persons skilled in the art may refer thereto with        the reservation that esterifications are excluded which are        conducted under conditions of acid catalysis. The preferred        conditions include activations of acid functions into ester        functions of 2-pyridinethiol, in the presence of        1-methyl-2-chloropyridinium iodide, in the presence of        dehydrating agents particularly chosen from the group:        dicyclohexylcarbodiimide (DCC), diisopropylcarbodiimide (DIPC)        combined with 4-methylamino-pyridinium tosylate (DPTS),        N,N′-carbonyldiimidazole, 1,1-carbonylbis(3-methylimidazolium),        alkylchloroformate and triethylamine, di-2-pyridylcarbonate, di        (2-pyridyl)-dithiocarbonate, 1-hydroxybenzotriazole,        penta-fluorophenyl diphenylphosphate, diethylazodicarboxylate        and triphenyl phosphine, Amberlyst-15,    -   for benzyls: on account of their better stability in acid        medium, in addition to the attack methods given above for        aryl-diazos, well-known methods may be used such as acid        catalysis (H₂SO₄, sulfonic para-toluene acid), distillation        and/or azeotropic distillation.    -   d) attack by an alcohol on an acrylate (transesterification        reaction):    -   for aryl-diazos: the embodiments containing acid catalysis are        to be excluded: on the other hand, the preferred treatment modes        include transesterification reactions in the presence of        catalysts such as titanates, tin oxides and esters such as        Bu₂Sn(OMe)₂, (Bu₂SnCl)₂O, dibutyl tin dilaurate, BuSn(O)OH, or        further Zn(CH₃COO)₂. Consideration may also be given to        conditions of basic catalysis in the presence of triethylamine        or 1,8-diazabicyclo[5.4.]undec-7-ene;    -   for benzyls: in addition to the reactions set forth above,        transesterifications under acid catalysis are possible; as        catalyst, special mention is made of sulfonic para-toluene acid,        sulfuric acid, boric acid, (pyro)phosphoric acid, phosphonic        acid, exchange resins such as DOWEX 50×2-100.        1°)2) The Nucleophilic agent is an —OCOR Oxycarbonyl Group, as        is the Case for:        the attack by salts of carboxylic acids on acryloyl halides in        the presence of pyridine or triethylamine or of exchange resins        carrying tertiary amino or quaternary ammonium groups, of Lewis        acids such as FeCl₃, AlCl₃ or a combination of these elements.        1°)3) The Nucleophilic Agent is a —S—R Group, According to        Embodiments Similar to Embodiments 1°1)a), 1°1)b), 1°1)c) and        1°1)d).        1°)4) The Nucleophilic Agent is a NR₂ Group as is the Case for:    -   a) attack by an amine according to an embodiment 1°1)a);    -   b) attack by an amine according to an embodiment 1°1)b);    -   c) attack by an amine according to an embodiment 1°1)c), in        particular the embodiments in the presence of a dehydrating        agent such as salts of pyridinium, of Bu₃N, or reactions        conducted in the presence of Ti(OBu)₄;    -   for benzyls: consideration may also be given to hot treatment        with acrylic acid in the presence of amides of carboxylic or        sulfonic acids, treatment in the presence of        trialkylaminoborane, in the presence of Lewis acids in        particular such as FeCl₃, ZnCl₂, potassium, sodium or ammonium        dihydrogen phosphate, BF₃-Et₂O;    -   d) attack by an ammonium salt of a primary amine with an        acrylamide optionally in the presence of complexing agents such        as BF₃;    -   e) attack by an amide on an acryloyl halide in the presence of a        base such as pyridine.        1°)5) The Nucleophilic Agent is a —NHCOR Group as is the Case        for Attack by an Amide on an Acryloyl Halide According to an        Embodiment 1°1)a).        2°) Second Variant: Grafting of Vinyl Units

In some embodiments of the invention, the polymerizable vinyl unit isalready present in its appropriate form in the molecule which graftsitself onto the photocleavable unit.

For this purpose, depending upon the type of vinyl unit, various methodsmay be used. However, having regard to the first mode of attachment ofthe acrylic function described above, the invention comprises a generalmode for grafting the vinyl function via all the mechanisms set forth inthe first mode of attachment, during which the acrylic function wascreated. This general mode assumes the reaction between a molecule or anorganic macrochain preferably having at one or more of its ends one ormore vinyl functions in a form that is the same as the one locatedwithin the desired structure of the photosensitive monomer, and havingat another end a reactive F1 function and a molecule or pre-synthesizedmacrochain carrying one or more aryl-diazo or benzyl units linked byvarious chemical skeletons, and on at least one end a reaction functionF2. (F2 is antagonist of F1 according to the first mode of attachment ofan acrylic function, i.e. under the conditions described above, F1 andF2 may react to form a covalent F1-F2 bridge).

A molecule including an aryl-triazene unit, linked by a suitablechemical skeleton to a —OH hydroxyl function, can for example be causedto react with a molecule having on its ends a polymerizable olefinfunction and a carboxylic acid function in the presence of a carboxylicacid function activator (DCC) used to graft the carboxylic acid onto thehydroxyl function.

In addition to all the grafting modes derived from the mechanismsdescribed above under 1°) for the first attachment mode of the acryloylfunction, two other types of reactions may be added.

2°)1) Formation of Carbamate Type Links, as is the Case for:

-   -   a) attack by a nucleophil on an isocyanate or isothiocyanate,        giving urethane or urea links for example in the case of attack        on an isocyanate by an alcohol or an amine respectively. Said        reaction may be commonly catalysed by catalysts such as DABCO or        metal-containing catalysts such as: Bu₂Sn(OMe)₂, dibutyl tin        dilaurate, BuSn(O)OH or Zn(CH₃COO)₂ for example.    -   b) The reaction of phosgene derivatives, such as a ROCOCI        chloroformiate on an alcohol or an amine or another nucleophilic        group. In this case a carbonate or urethane may be obtained        according to a preferred nucleophilic catalysis mode as in        1°1)b).    -   c) Reactions of transesterification type conducted between a        nucleophilic attacker of alcohol or amine type for example on a        carbamate precursor of phenylcarbonate or phenylurethane type        for example, said reaction being preferably catalyzed in the        presence of a catalyst of the type cited previously under        2°)1)a).        2°)₂) Formation of β-Hydroxyester Links        such as those obtained in particular by nucleophilic attack by a        carboxylate anion on an oxirane; this reaction preferably being        conducted under nucleophilic catalysis in the presence of        catalysts such as triethylamine, N-methylmorpholine,        N-methyl-pyrrolidine, N,N-dimethylbenzylamine, ion exchange        resins carrying tertiary amine or quaternary ammonium sites and        more particularly referenced as weak and/or strong base anion        exchange resins such as Dowex 44, these catalysts being used        either alone or in the presence of co-catalysts such as a Lewis        acid, for example FeCl₃ or CrCl₃, or of Zn(OOC—C₇H₁₅)₂ or        BF₃O(C₂H₅)₂ type, however acid catalysts may be considered for        o-nitrobenzyls (for example trifluoroacetic acid).

Case 2: Polymerizable Units of Oxirane Type

Here consideration will be given to all the above-described oxiraneunits, namely those which can be cationically polymerized (formula V)and anionically polymerized.

As for the vinyl units, the two variants for linking an oxirane unit toa photocleavable unit are either the creation of an oxirane unit on theskeleton of the photocleavable unit, or grafting.

1°) First Variant: Creation of an Oxirane Function

One first way of creating an oxirane function consists of treating ahalohydrine function according to a nucleophilic substitution method ofsecond order, using a base such as NaOH for example in the presence ofwater or dimethylsulfoxide for example. The dehydration may also beperformed of a 1,2-diol function by treatment with DMF dimethylcetal ordiethyl azodicarboxylate in the presence of PPh₃.

A second method, preferably applied to units of benzyl type for which itis a method of choice, consists of epoxidation of an olefin in thepresence of a peroxydic reagent, among which preference is given tom-chloroperoxybenzoic acid or 3,5-dinitroperoxybenzoic acid. The use ofhydrogen peroxide or tert-butyl hydroperoxide in the presence of analkaline solution or the use of Triton B (PhCH₂N⁺Me3OH⁻) is also apreferred manner for benzyl or aryltriazene substrates, or further thetreatment of olefins with an alkyl peroxide and catalyzed by a Vanadium,Titanium or Cobalt complex. For benzyl units having at one end of thechemical skeleton a unit of allylic alcohol type, it is possible toconduct synthesis of an oxirane function in the presence of tert-butylhydroperoxide, titanium tetraisopropoxide and a dialkyl tartrate underSharpless conditions, or further with the combination ofRe₃O/bis(trimethylsilyl)peroxide/H₂O₂.

A third manner applicable to benzyl and aryldiazo photocleavable unitsis the treatment of an alcoholate or phenate, generated at one end ofthe skeleton linked to the photocleavable unit, with an epihalohydrineor any similar derivative. A derivative method is treatment of acarboxylic acid, or preferably its salt—sodium for example, or any otherequivalent function such as sulfonic acids for example, with anepihalohydrine in the presence of quaternary ammonium salts for examplesuch as benzyltrimethylammonium chloride or further in the presence ofion exchange resins referenced as weak base/high base anion exchangeresins.

A fourth manner applicable to photocleavable benzyl and aryldiazo units,preferably to aryltriazenes, is treatment of an aldehyde or ketonehaving at least one end of the skeleton linked to the photocleavableunit by a sulfur ylide such as dimethyloxosulfonium methylide ordimethylsulfonium methylide.

2°) Second Variant: Grafting of Oxirane Units

Several, non-limitative, examples are given below to conduct thisgrafting.

One first example, set forth above under 1°) but applicable here sinceit involves destruction of an oxirane function, that is present on areagent, for its subsequent re-creation during the reaction, is thetreatment of an alcoholate or phenate, generated on at least one end ofthe skeleton linked to the photocleavable unit, with an epihalohydrine.The alcoholate may be generated for example in the presence of sodiumhydride or by using solutions, in particular aqueous solutions of NaCHor KOH, in the optional presence of a tertiary amine (for exampletriethylamine, tributylamine) or an ammonium salt (tetrabutylammoniumbromide for example) or an ion exchange resin. A derivative method isthe treatment of a carboxylic acid or preferably its salt, sodium forexample, or any other equivalent function such as sulfonic acids forexample, with an epihalohydrine in the presence of quaternary ammoniumsalts for example, such as benzyltrimethylammonium chloride, or of ionexchange resins referenced as weak base/strong base anion exchangeresins.

A second method consists of treating an alcohol with epichlorhydrine, orany epihalohydrine or any 1,2-epoxy derivative, in the presence of acatalyst of aluminium trialkoxide type, preferably C1-C4, or ofaluminium isopropoxide type for example and of an acid co-catalyst suchas H₂SO₄ for example followed by removal under alkaline conditions.

A third manner is the treatment of an alcohol with an epihalohydrine inthe presence of ZnCl₂/H₂SO₄ or ZnCl₂/BF₃,Et₂O followed by removal underalkaline conditions. SnCl₄ or BF₃,Et₂O are also reagents which can beused under similar conditions.

A fourth manner is the treatment of benzyl halide groups (carried by theskeleton of the photocleavable unit) with glycol or its derivatives inthe presence of a hydride, in particular sodium hydride, in a solventsuch as THF, acetonitrile or DMF, optionally in the presence of aquaternary ammonium salt such as tetrabutylammonium iodide for example.

Below are given some examples of bifunctional monomers incorporatingphotocleavable and polymerizable units which meet the above citeddefinitions and are synthesized according to any of the aforesaidtechniques. Structure Name Ref

1,2-Bis[1-(4″- mediacrylatemethyl-)- phenyl-3- methyl]triaz(1)ene-ethaneAT1

1,2-Bis[1-(4′- (methacrylate-ethyl) aminocarbonyloxymethyl)phenyl-3-methyl- ]triaz(1)ene-ethane AT2

1-(4′-(methacrylate- ethyl)aminocarbonyloxym ethyl)phenyl-3-((methacrylate- ethyl)aminocarbanyloxyet hyl)-3-methyt-triaz(1)ene AT4

1-(4′-methacrylatemethyl- )phenyl-3,3-di(2″- methacryolylethyl)-triaz(1)ene AT5

1-(4′-(methacrylate- ethyl)aminocarbonyloxym ethyl)phenyl-3,3-di(((methacrylate- ethyl)aminocarbonyloxyet hyl)-triaz(1)ene AT6

1-(3′-(methacrylate-ethyl carboxyphenyl)-3-di(2″- methacrylate-ethyl)triaz(1)ene AT7

1,2-Bis[1-(3″- methacrylate- ethylcarboxyphenyl)-3-methyl]triaz(1)ene-ethane AT8

1-(3′-ethyl glycidyl ether carboxyphenyl)-3-(ethyl glycidylether)-3-methyl- triaz(1)ene AT9

1-(3′-ethyl glycidyl ether carboxy-6′-methylphenyl)- 3-(ethyl glycidylether)-3- methyl-triez(1)ene AT10

1-(4′-methyl glycidyl ether)-3-(ethyl glycidylether)-3-methyl-trlaz(1)ene AT11

2-methacrylatemethyt-5-(3- (2″methacrylate-ethyl)-3′-methyl)triaz(1)ene- thiophene AT12

1,5-bis[4′- (methacrytatemethyl)pheny lazomethayl-phosphonate]-diethylene glycol AAP1

1,5-bis[4′-(methyl glycidyl ether)phenylazomethyl-phosphonate]-diethylene glycol AAP2

2-Methyl-acrylic acid 5- methoxy-4-[2-(2-methyl- acryloyloxy)-ethoxy]-2-nitro-benzyl ester NT1

2-Methyl-acrylic acid 1-{5- methoxy-4-[2-(2-methyl-acryloyloxy)-ethoxy]-2- nitro-phenyl}-ethyl ester NT2

2-Methyl-acrylic acid 4,5- bis-[2-(2-methyl- acryloyloxy)-ethoxy]-2-nitro-benzyl ester NT3

2-Methyl-acrylic acid 2-(5- methoxy-4-{2-[2-(2- methyl-acryloyloxy)-ethoxycarbonyloxy]- ethoxy}-2-nitro- enzyloxycarbonyloxy)- ethyl esterNT4

2-[2′-nitro-4′,5′- di(oxymethyloxirane)]benz yloxymethyloxirane NT5

(2-Methoxy-5-nitro-4- oxiranylmethoxymethyl- phenoxy)-acetic acidoxiranylmethyl ester NT6

In a particularly advantageous variant of the invention, thebifunctional monomer is of oligomer or prepolymer size, optionally hasmore than two polymerizable units, and has a defined so-called“controlled” complex structure, i.e. the conditions and parameters ofsynthesis of these particular compounds enable the desired structure tobe obtained.

The use of said bifunctional monomers in the adhesive composition of theinvention offers advantages in respect of control over the viscosity ofthe composition and can reduce the well-known phenomenon ofpolymerization shrinkage and its disadvantages.

Two aspects are presented more particularly below which are found in theexamples of bifunctional compounds and adhesive compositions of theinvention.

A—According to a first aspect of this advantageous variant, thebifunctional monomer is a linear polymer having a branched combstructure in which the photocleavable units and the polymerizable unitsare preferably arranged on the comb branches of the principal linearchain of the polymer. Said bifunctional monomer may schematically bedescribed as follows:

where

is a photocleavable unit chosen from among the above-describedphotocleavable units or any other photocleavable unit photosensitive toa wavelength of λ2 and

is a polymerizable unit chosen from those described above.

In a first preferred alternative, the principal linear polymer chain isa chain whose structure may be chosen from among a large variety ofpossible structures, such as poly(ether), poly(ester), poly(acrylate),poly(amide), poly(acrylamide), poly(imide), poly(styrene),poly(urethane), poly(carbonate), poly(siloxane), poly(epoxide),poly-(phtalamide), poly(ethersulfonate), poly(alcene),poly-(aryltriazene), poly(arylasophosphonate), poly(o-nitrobenzyl) andany other conventional skeleton in polymerization techniques that iscompatible with optional structural modification to allow grafting ofthe chain or the substituents contained in the chemical skeletons tolink at least one photocleavable unit to at least one polymerizableunit, the bond between the photocleavable unit or units and thepolymerizable unit or units being achieved in a way that conforms todefinitions of bifunctional monomers of the invention, capable underradiation of wavelength □2 to photo-disrupt the principal branches ofthe linear chain of the polymer which therefore frees itself of thecrosslinked polymer network obtained after polymerization of theadhesive composition.

In a second alternative, the branches comprising one or morephotocleavable units and one or more polymerizable units may besynthesized in two or three steps, similarly to the general synthesisschema for bifunctional monomers set forth above, for example synthesisof the photocleavable unit directly on the pluri-substituted orpoly-branched polymer chain followed by structural modification and/orfollowed by attachment of the polymerizable units, or grafting of thepre-synthesized photocleavable unit and attachment of the polymerizableunits for example, or grafting of the “photocleavable unit-polymerizableunit” entity.

Advantageously, in a preferred embodiment of the two precedingalternatives, the principal linear chain can have substituents or can bemodified so as to be provided with substituents able to set up acovalent bond with a chain whose chemical skeleton is made up of atleast one photocleavable unit linked to at least one polymerizable unit,the achievement of this covalent bond being made using the synthesismodes described above for grafting vinyl polymerizable units.

A-a) In a particular embodiment, the branches have polymerizablefunctions of vinyl type at their ends.

One preferred manner to conduct grafting is “in one pot” synthesis inwhich the attachment of the vinyl unit is immediately followed bygrafting of the “photocleavable unit(s)-vinyl unit(s)” branch, the twosuccessive steps being based on the grafting reactions describedpreviously for vinyl units. More precisely, the skeleton of thephotocleavable unit terminates in two antagonist functions, for exampleone of these functions is typically —OR, —OCOR, NR₂ whilst the otherfunction is essentially an acid or carboxylic ester.

The method for preparing said monomer consists of initially conductingthe synthesis or grafting of a vinyl function through a nucleophilicsubstitution reaction on an acyl carbon. For an acid function, thereaction is preferably conducted in the presence of dehydrating agentssuch as 2-pyridinethiol, 1-methyl-2-chloropyridinium iodide,dicyclo-hexylcarbodiimide, N,N′-diisopropylcarbodiimide,N,N′-carbonyldiimidazole, 1,1′carbonylbis(3-methylimidazolium)triflate,di-2-pyridyl carbonate, 1-hydroxybenzotriazole, an acylation agent ofPyridine/Tosyl Chloride type or SOCl₂/DMF, preferably1-methyl-2-chloropyridinium iodide, 2-pyridinethiol,dicyclohexylcarbodiimide, N,N′-diisopropyl-carbodiimide. For an esterfunction, the reaction is a transesterification that is preferablyconducted in the presence of catalysts such as titanates, organic tinoxides and esters such as those cited above, using a basic catalysismethod in the presence of non-ionic bases providing soft operatingconditions such as amines (for example triethylamine,1,2,2,6,6-pentamethyl-piperidine, 4-dimethyl-aminopyridine), amidines(e.g. 1,8-diazabicyclo[5.4.0]undec-7-ene), guanidines (e.g.1,5,7-triazabicyclo[4.4.0]dec-5-ene, 1,1,3,3-tetramethyl-guanidine,1,3-diphenylguanidine), triamino(imino)-phosphoranes; in preferredmanner in the presence of guanidine derivatives, bisguanidines inparticular as described in Tetrahedron Letters 39 (1998), 2743-2746),soluble polyguanidines or supported on a silica or polystyrene supportdescribed in Reactive & Functional Polymers 48 (2001), 65-74.

In a second step, the entity consisting of the photocleavable unit/vinylunit obtained after the first step is grafted to a substituent of theprincipal linear polymer chain, said substituent ending in a functionantagonist to an end function of the photocleavable unit/vinyl unitentity. The operating conditions are similar to those for the first stepof the method.

A-b) In a further particular embodiment, the branches have polymerizablefunctions on their ends of vinyl or oxirane type.

A preferred method of grafting is three-step synthesis: the first stepconsists of grafting the skeleton of the photocleavable unit onto thesubstituents of the principal chain, the second is the structuralmodification of the grafted chemical skeleton (or graft), the third isthe attachment of the polymerizable unit through creation or grafting.According to this embodiment, the grafting of the first step is thecreation of a covalent bond using the same synthesis modes as set forthpreviously for the grafting of vinyl units. The structural modificationof the second step is a conversion causing an “antagonist” function toappear on the end of the graft (such as previously designated F1 and F2)enabling the creation or grafting during the third step of polymerizableunits according to the general synthesis modes described above for vinyland oxirane units, or according to the more particular modes given underA-a) in the first particular embodiment of this first aspect of thisadvantageous variant of the invention.

The preferred conversions of the second step are the general reducingmethods of carbon-heteroatom unsaturated derivatives (O or N) by carbonor heteroatom attack making it possible to create “antagonist” functionsprovided that they are compatible with the chemical skeletons (polymerchain, photocleavable unit, etc.). Reagents of choice are LiAlH₄, NaBH₄,AlH₃₁NaBH₄ for example in isopropanol, sodium triacetoxyborohydride,Li(Et₃CO)₃AlH, ZnBH₄ in THF, or NaBH₄ in aqueous ethanol in the presenceof CeCl₃. Depending upon the type of polymer chain, aluminiumisopropoxide in isopropanol is a method applicable to ketone reduction.The reagents derived from boranes such as BH₃-THF, BH₃-triethylamine,BH₃-Me₂S, 9-BBN are generally adapted in most cases to the reduction ofcarbonyl compounds to obtain alcohol functions in the presence ofphotocleavable units such as aryltriazene or 2-nitrobenzyl.

The previously cited reagents, and more generally the derivatives ofmetallic hydrides, properly selected, may therefore be used to createalcohol or amine functions, from imine functions, Schiff bases, nitrile,epoxide, carboxylic acid derivatives with the exclusion of the nitrogroups if a 2-nitrobenzyl function is present (therefore LiAlH₄ cannotbe used in this instance).

Other conversions are possible for the second step:

-   -   the generation of “antagonist” functions may also be made,        depending upon the type of all the chemical skeletons present,        using organometallic compounds added onto functions such as        carbonyl, unsaturated α, β carbonyl, imine, acid and        derivatives, or oxirane. The preferred reagents are: Grignard        reagents optionally in the presence of LiOCl₄, Bu₄N⁺Br⁻,        toluene, benzene, CeCl₃, TiCl₄, (RO)₃TiCl, (RO)₃ZrCl, (R₂N)TiX,        in particular when special functions are present; alkyl- or        aryllithiums, alkylzincs, tin allyltrialkyls, in the presence of        BF₃, Et₂O, allyltrialkysilanes in the presence of a Lewis acid,        allyl boranes; the conditions of the Reformatsky reaction        (ketone or aldehyde, □-halo ester) in the presence of zinc.    -   hydroboration of olefins is a particularly preferred conversion,        preferably on the ends of the skeletons of the photosensitive        grafts, which enables soft generation of alcohol functions after        hydrolysis in the presence of NaOH and hydroperoxide. Suitable        reagents are BH₃-THF, BH₃-Me2S or NaBH₄ combined with BF₃, Et₂O,        9-BBN, mono/dialkylboranes.    -   one applicable conversion depending upon the type of chemical        skeletons, is hydrolysis of imines, Schiff bases or isocyanates,        optionally with acid or basic catalysis, to generate an amine        function at the end of the photocleavable grafts.    -   one particularly preferred conversion to obtain a primary or        secondary amine function consists of converting a halide        derivative of 2-nitrobenzyl into aminomethyl-2-nitrobenzene for        example by treatment with potassium phtalimide followed by        hydrazinolysis, or into N-substituted aminomethyl-2-nitrobenzene        by substitution in the presence of the corresponding        N-substituted amine.

Examples of such bifunctional prepolymer compounds, of polysubstitutedlinear polymer type, are:

-   Poly((14-(2′-aminoacylethyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(2′4′-aminoacylethyl)-1,4-dioxa-5-oxo-6-aza-heptane)),-   Poly[(0.14-(4′-aminoacylethexyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(4′-aminoacylhexyl)-1,4-dioxa-5-oxo-6-aza-heptane)],-   Poly[(14-(4′-′    (4″-aminocylphenyl)methylphenyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(4′-(4″-aminoacylphenyl)methylphenyl)-1,4-dioxa-5-oxo-6-aza-heptane)),-   Poly((14-′4′-(4″-aminoacylcyclohexyl)methylcyclohexyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(4′-(4″-aminoacylcyclohexyl)methylcyclohexyl)-1,4-dioxa-5-oxo-6aza-heptane)]    -   in which all these polymers are esterified on the hydroxy group        at position 6 of the copolymer chain by groups of type:-   -oxycarbonyl-3-[3′-(2″-(methacrylate)ethyl)]-3′-methyl-triazene)    phenyl-   oxycarbonyl-ethyloxy-(1-methoxy-3-(methacrylatemethyl)-4-nitro)phenyl.

Preferred compounds of polysubstituted linear polymer type are:

-   Poly[(14-(4′-methylaminoacylcyclohexyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(4′-methylaminoacylcyclohexyl)-1,4-dioxa-5-oxo-6-aza-heptane)],-   Poly[(14-(4′-aminoacylbutyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(4′-aminoacylbutyl)-1,4-dioxa-5-oxo-6-aza-heptane)],    -   both being esterified on the hydroxy group at position 6 of the        copolymer chain by groups of type:        -oxycarbonyl-3-[3′-(2″-(methyacrylate)ethyl))-3″methyltriazene]phenyl        or of type        -oxycarbonyl-ethyloxy-(1-methoxy-3-(methacrylatemethyl)-4-nitro)phenyl.

These four preferred associations will respectively be referred to as:PU1AT1, PU1NT1, PU2AT1 and PU2NT1.

B—According to a second aspect of the variant in which the bifunctionalmonomer is of oligomer of prepolymer size, the bifunctional monomer is ahyperbranched polymer whose structure derives from step polymerizationof a type AB_(n) monomer, and is preferably derived from steppolymerization of a monomer of type AB₃ or AB₂, where A and B arerespectively two different chemical functions capable of reactingtogether under condensation or addition reaction to give a polymer chainhaving branch point where every B function disappears.

In a first alternative, the hyperbranched polymer so designated has a“core” consisting in particular of monomer units having one or morephotocleavable units in their skeletons and a “shell” around the corewhose branches are formed of essentially inert monomer units from aphotochemical viewpoint, the ends of the branches ending in at least onepolymerizable unit, preferably only one.

In a second alternative, the hyperbranched polymer so designated has a“core” formed in particular of essentially inert monomer units from aphotochemical viewpoint and a peripheral “shell” around the core whosebranches consist of monomer units having one or more photocleavableunits in their skeleton and whose branch ends terminate in at least onepolymerizable unit, preferably only one.

The preferred general structure of a type AB₂ monomer able to polymerizefollowing a step polymerization mode to give a hyperbranched polymersuch as designated is as follows:

The functions A and B meet the criterion of antagonist functions and, inthis respect, may be chosen from among all those previously described.In preferred manner, A is a function derived from carboxylic acid,preferably a carboxylic acid, carboxylic ester, carboxylate,trimethylsilylated carboxylic acid, acyl chloride function, and B is analcohol function, optionally trimethylsilylated, esterified, a NH₂ aminogroup or a primary amine or primary ammonium salt. The preferredcombinations are those in which A is a carboxylic acid function or esterand in which B is an alcohol or amino function.

Typical examples of AB₂ monomers are for example2,2-bis(hydroxymethyl)propionic acid (inert monomer) and1-(3′-carboxyphenyl)-3-,3-di(2″-hydroxyethyl)triazene (photosensi-tivemonomer).

The privileged preparation modes for these hyper-branched structures arebased on the use of dehydrating agents, in particular if A is acarboxylic acid function, such as in particular1-methyl-2-chloropyridinium chloride, dicyclohexylcarbodiimide,N,N′-diisopropylcarbodiimide, N,N′-carbonyldiimidazole,1,1-carbonylbis(3-methyl-imidazolium) triflate, di-2-pyridyl carbonate,1-hydroxybenzotriazole, an acylation agent of Pyridine/Tosyl Chloridetype or SOCl₂/DMF, preferably 1-methyl-2-chloropyridinium chloride,2-pyridinethiol, dicyclo-hexylcarbodiimide,N,N′-diisopropyl-carbodiimide. If A is an ester function, the reactionis a transesterification preferably conducted in the presence ofcatalysts such as titanates, organic tin oxides and esters (inparticular those already cited), using a basic catalysis mode in thepresence of non-ionic bases providing soft operating conditions such asamines (for example triethylamine, 1,2,2,6,6-pentamethylpiperidine,4-dimethylaminopyridine), amidines (for example1,8-diazabicyclo[5.4.0]undec-7-ene), guanidines (for example1,5,7-triazabicyclo[4.4.0]dec-5-ene, 1,1,3,3-tetra-methylguanidine,1,3-diphenylguanidine), triamino(imino) phosphoranes; in preferredmanner, in the presence of the guanidine derivatives, bisguanidines inparticular such as described in Tetrahedron Letters 39 (1998),2743-2746, and polyguanidines either soluble or supported on a silica orpolystyrene support as described in Reactive & Functional Polymers 48(2001), 65-74.

Particular mention is made of the case in which the photocleavable unitsused are exclusively benzyl derivatives and more specifically2-nitrobenzyl derivatives. In this case, the step polymerizationreactions, in particular the polyesterification reactions, areadvantageously conducted under conditions of acid catalysis (in thepresence of sulfonic para-toluene acid or sulfuric acid for example), inaddition to the preceding conditions which also apply.

As indicated at the beginning of this part concerning the privilegedvariant of bifunctional monomers, the structures are preferablycontrolled which means that for this second aspect a distinction is madebetween the preparation of the core and of the peripheral shell. Inparticular when the “core” is a photocleavable core, the preparationmethods are based on step polymerization methods by slow addition of theAB₂ monomers to a system of “core molecules” present in strongconcentration in the reaction medium at all times relative to theconcentration of reactive AB₂ monomers. These preparation methods, orderivative methods, are described in particular in Macromolecules, 1998,31, 3790-3801; Macromolecules, 2001, 34, 7692-7698; Macromolecules,2002, 33, 3212-3218; Macromolecules, 2000, 33, 3099-3104; Polymers forAdvanced Technologies, 2001, 12, 346-354. Advantageously the core istypically a photocleavable molecule of B_(n) type and includes functionssuch as aryltriazenes or 2-nitrobenzyls, or is a hyperbranched polymerchain such as the one obtained by polycondensation of1-(4′-carboxyphenyl)-3,3-di(2″-hydroxy-ethyl)triazene, of1-(3′-carboxyphenyl)-3,3-di (2″-hydroxyethyl)triazene, or of1-(3′-carboxy-6′-methylphenyl)-3,3-di-(2″-hydroxyethyl)triazene in thepresence of N,N-diisopropylcarbodiimide and APTS.

The preferred preparation methods use the attachment of polymerizableunits to branch ends to give the bifunctional monomer of the invention,according to a final step in which the polymerizable unit is created orit is grafted by attachment of a final photosensitive or inert layer oftype AB_(n) monomers, which already carry polymerizable units. Thisfinal step is performed using all the possibilities indicated previouslyfor the synthesis process according to the compatibilities offered bythe chemical structure of the synthesized polymers, in which preferenceis given to:

-   -   for vinyl units: possibilities 1°)1) to 1°)₄) for creation, all        of 20) for grafting;    -   for oxirane units: the first three manners for creation and the        first and third manner for grafting.

For the first alternative of bifunctional structures of hyperbranchedpolymer type, namely with photosensitive core and inert shell, thefollowing particular examples are chosen:

-   Poly(1-(3′-carboxyphenyl)-3-,3-di(2″-hydroxyethyl)triazene),-   Poly(1-(3′-carboxy-6′-methylphenyl)-3-,3-di(2″-hydroxyethyl)triazene),-   Poly(1-(4′-carboxyphenyl)-3-,3-di(2″-hydroxyethyl)triazene),-   Poly(1-(3′,5′dicarboxypheny)-3-(2″-hydroxyethyl)-3-methyl-triazene),

ω-functionalized by methacrylate ends with methacrylic acid and itsderivatives, such as 2-hydroxyethylmethacrylate, glycidyl methacrylateor 2-isocyanatoethyl methacrylate for example, or by oxirane ends ofglycidyl type by reaction with an epihalohydrine, for example.

-   Poly(1-(3′carboxyphenyl)-3-,3-di(2″-hydroxyethyl)triazene),    α-functionalized by the derivatives of methacrylic acid and which    will be denoted PH1AT1.-   Poly(1-(3′-carboxy-6′-methylphenyl)-3-,3-di(2″-hydroxyethyl)triazene),    co-functionalized by the derivatives of methacrylic acid which shall    be denoted PH1AT2.

The above examples of hyperbranched structures are among the most simplewhich may be considered, the inert shell consisting of polymerizableunits.

Other examples of hyperbranched structures with photosensitive core andinert shell are in particular:

-   Poly(1-(3′carboxyphenyl)-3-,3-di(2″-hydroxyethyl)triazene-co-2,2-bis(hydroxymethyl)propionic    acid),-   Poly(1-(3′-carboxy-6′-methylphenyl)-3-,3-di(2″-hydroxy-ethyl)triazene-co-2,2-bis(hydroxymethyl)propionic    acid),-   Poly(1-(4′-carboxyphenyl)-3-,3-di(2″-hydroxyethyl)triazene-co-2,2-bis(hydroxymethyl)propionic    acid),

ω-functionalized by methacrylate ends with methacrylic acid and itsderivatives, such as 2-hydroxyethylmethacrylate, glycidyl methacrylateor 2-isocyanatoethyl methacrylate for example, or by oxirane ends ofglycidyl type by reaction with an epihalohydrine, for example

-   Poly(1-(3′-carboxyphenyl)-3-,3-di(2″-hydroxyethyl)triazene-co-2,2-bis(hydroxymethyl)propionic    acid, ω-functionalized by the derivatives of methacrylic acid and    denoted PH2AT1;-   Poly(2,5-Bis-chloromethyl-1,3-dinitro-benzene-co-2,2-bis(hydroxymethyl)propionic    acid) ω-functionalized by the derivatives of methacrylic acid and    denoted PH2NT3.-   Poly((2-Nitro-4,5-bis-oxiranylmethoxy-phenyl)-methanol-co-2,2-bis(hydroxymethyl)propionic    acid) ω-functionalized by the derivatives of methacrylic acid, and    denoted PH2NT1.

For the second alternative of bifunctional structures of hyperbranchedpolymer type, namely with inert core and photosensitive shell,particular choice is made of:

-   Poly(2,2-bis(hydroxymethyl)propionic acid    -co-1-(3′-carboxyphenyl)-3-, 3-di (2″-hydroxyethyl)triazene),-   Poly(2,2-bis(hydroxymethyl)propionic acid    -co-1-(3′-carboxy-6′-methyhenyl)-3-,3-di(2″-hydroxyethyl)triazene),

ω-functionalized by methacrylate ends with methacrylic acid and itsderivatives, such as 2-hydroxyethylmethacrylate, glycidyl methacrylateor 2-isocyanatoethyl methacrylate for example, or by oxirane ends ofglycidyl type by reaction with an epihalohydrine, for example.

-   Poly(2,2-bis(hydroxymethyl)propionic acid), (ω-functionalized by an    oxycarbonyl-3-[3′-(2″-(methacrylate)ethyl))triazene]phenyl group or    an    -oxycarbonyl-ethyloxy-(1-methoxy-3-(methacrylatemethyl)-4-nitro)phenyl    group.

DETAILED EXAMPLES OF BIFUNCTIONAL COMPOUND SYNTHESIS

Below is a detailed description of four examples implementing the methodof fabricating bifunctional monomers of the invention, such as describedpreviously in more general manner. On the basis of these examples ofsynthesis and proposed techniques, those skilled in the art will be ableto obtain any photocleavable, polymerizable bifunctional monomer meetingthe minimum required structural criteria for producing thephotosensitive adhesive composition of the invention.

1. Synthesis of the NT5 Bifunctional MonomerStep 1:

To a single neck, oven-dried, 100 mL round-bottomed flask, are added 2 g6-nitropiperonal, then under inert atmosphere at 0° C., 30 mL BBr₃ (1.0Min CH₂Cl₂). The medium is then stirred at ambient temperature. 10 mLBBr₃ (1.0M) are further added after 12 h and 5 mL after 24 h. After 48h, the reaction medium is decanted into a 1 litre bottle placed over anice bath and 100 mL water are added very slowly. The mixture isconcentrated. The residue is then collected with minimum volume THF andagain cold precipitated by adding 35% hydrochloric acid. The precipitateis filtered through a glass sinter filter and the operation is repeatedat least 3 times. The precipitated product is finally purified by flashchromatography on silica gel with an eluant (70 mL/30 mL/5 mL petroleumether/ethyl acetate/ethanol). 1.28 g of compound NT5.a are obtained.Step 2:

To a three neck, oven-dried round-bottomed flask, 1 g of NT5.a compoundare added and 516 mg NaBH₄. The flask is surmounted by a refrigerant andthe assembly closed and placed in an inert atmosphere. Using a syringe,100 mL anhydrous THF are added. The reaction medium is stirred for 30 hat ambient temperature. At the end of the reaction, 20 mL ethanol areadded and left under stirring for 30 minutes. The reaction medium isconcentrated, collected several times in THF and filtered. The groupedfiltrates are concentrated and the residue is purified by flashchromatography on silica gel with eluant (70 mL/30 mL/10 mL petroleumether/ethyl acetate/ethanol). 960 mg of compound NT5.b are obtained.Step 3:

To a 200 mL three neck round-bottomed flask, 16 mL aqueous solutioncontaining 50% potassium hydroxide and 11 ml epichlorhydrine are addedtogether with 380 mg tetrabutyl-ammonium bromide. The mixture is stirredat 0° C. and followed by cold, progressive addition of 1.5 g NT5.b sothat the temperature does not exceed 25° C. After a reaction time of 15h, the medium is poured into 30 mL of a water/ice mixture. The aqueousphase is extracted with ethyl ether (3*60 mL). The organic phases aregrouped together and washed with an aqueous solution of NaCl (5*60 mL),dried on sodium sulfate and concentrated. The residue is distilled underreduced pressure and purified by flash chromatography on silica gel withan eluant (90 mL/10 mL petroleum ether/ethyl acetate) to yield 1.26 g ofcompound NT5.

II. Synthesis of the AT1 Bifunctional Monomer

Step 1:

-   -   a) The diazonium tetrafluoroborate salt of 4-ethyl-aminobenzoate        was prepared (hereafter denoted AT1.a):

To a three neck, oven-dried round-bottomed flask, 15 g ethyl4-aminobenzoate are added in 300 mL dry CH₂Cl₂ and 50 mL dry THF. Themedium is placed in an inert atmosphere and the flask is immersed in abath cooled to −15° C. 19.35 g BF₃,Et₂O are then slowly added in 40 mLdry CH₂Cl₂. When thermal equilibrium is reached 12.5 g tert-butylnitrite is added over 1 h30 in 80 mL dry CH₂Cl₂ and the medium ismaintained under vigorous stirring. When addition is completed, themedium temperature rises to 0° C. Approximately 150 mL pentane are thenadded to the reaction medium. The formed precipitate is filtered, thenwashed with 300 mL ethyl ether, filtered again and finally dried at 25°C. in a drying oven in the presence of P₂O₅. 23.1 g of compound AT1.aare obtained.

-   -   b) Diazoic coupling was performed and a compound hereafter        designated AT1.b is obtained:

In 250 mL dry MeCN, 15.4 g AT1.a is dissolved and 12.82 g sodiumcarbonate is added. The medium is cooled in an inert atmosphere over abath at −4° C. 4.4 g N,N′-dimethylethylene-diamine are added to anaddition funnel in 160 mL dry MeCN in the presence of sodium carbonate.The addition to the reaction medium is conducted over 6 h under vigorousstirring. When addition is completed, stirring is maintained for 1 h 30.The reaction medium is then filtered and the salt is washed in ethylacetate. The product is isolated by successive dissolutions of the cruderesidue in a minimum volume of ethyl ether, cold crystallization andfiltration of the precipitate. The product is dried in a drying oven inthe presence of P₂O₅ and 12.03 g of the compound AT1.b are obtained.Step 2:

One half-spoon LiAlH₄ is placed in a three neck round-bottomed flask,previously oven-dried and fitted with a refrigerant and an additionfunnel. The assembly is placed in an inert atmosphere and immersed in anoil bath at 55° C. 4 g AT1.b compound dried on P₂O₅ are previouslyplaced in the addition funnel. 40 mL dry THF are then added to the flaskand 100 mL to the addition funnel. The AT1.b compound is then added dropby drop to the flask under vigorous stirring, and gradually, 120 mL dryTHF and 1.8 g LiAlH₄ divided into several fractions are directly addedto the reaction medium. The medium is stirred at 50° C. for 3 h. At theend of the reaction, a mixture of 50 mL THF and 50 mL ethyl acetate isadded slowly, the crude is passed through the glass sinter and thecollected solid is washed in methanol until it becomes white. Thefiltrates are concentrated, washed in a mixture of methanol and THF, andthe recrystallized solid is filtered through a sinter. These operationsare repeated until practically all the aluminates have been drawn fromthe AT1.c compound of which 3.23 g is obtained.Step 3:

To an oven-dried three neck round-bottomed flask surmounted by arefrigerant, are added 1.219 g AT1.c compound dried on P₂O₅ and 2.5 g4-dimethylaminopyridine (DMAP). The medium is placed in an inertatmosphere and brought to 50° C. Using a dropping funnel, 150 mL dry THFare added to the reaction medium under stirring. After 5 minutes, asyringe is used firstly to add 1.55 mL methacrylic anhydride with +0.3weight % hydroquinone, then 50 mL dry THF in the dropping funnel. Theadditions are made slowly to the mixture for one hour and thetemperature is then maintained at 50° C. for a further hour, thenstirring is continued for 12 h at 20° C.

The reaction medium is then extracted with THF and is cold concentrated.The residue is then extracted by a succession of biphase separations.The crude is first dissolved in 125 mL CH₂Cl₂. This first phase isextracted with 150 mL iced water, which is in turn extracted with 50 mLCH₂Cl₂.

The operation is repeated three times. After grouping the organic phasestogether and drying on sodium sulfate, they are evaporated and theresidue is purified by flash chromatography on silica column with aneluant (85 mL/4 mL/13 mL petroelum ether/ethyl ether/ethyl acetate).1.53 g of compound AT1 are obtained.

III. Synthesis of the PU1AT1 Bifunctional MonomerStep 1:

To an oven-dried, three-neck round-bottomed flask fitted with a magneticstirrer and surmounted by a refrigerant, 450 mL distilled anhydrousglycol ethylene are added under an inert atmosphere. 14 mL3-chloro-2-chloromethyl-1-propene are added using a syringe. The flaskis immersed in an oil bath at 60° C., under vigorous stirring. 12 g 95%NAH are prepared in several portions and the entire quantity of NaH iscarefully added in small successive portions to the reaction medium.When addition is completed, the bath temperature is set at 80° C. and itis left under stirring for 24 h.

At the end of the reaction, the mixture is distilled under reducedpressure of 4 mm Hg at a temperature of 80° C. to 140° C. so as toremove a maximum amount of ethylene glycol. The residue is purified byflash chromatography on silica gel with eluant (85 mL/15 mL/2-10 mLpetroleum ether/ethyl acetate/ethanol). The product is againconcentrated by distillation under reduced pressure and 16.8 g ofcompound PU1AT1 is obtained with 57.8 weight percentage in a mixturewith residual ethylene glycol.Step 2:

In an oven-dried, single neck round-bottomed flask, a magnetic stirreris placed with 3.555 g of a previously obtained and freshly distilledPU1AT1.a/ethylene glycol mixture. After desolvation under reducedpressure of 4 mmHg for 15 h, the medium is placed in an inertatmosphere. Using a syringe, 5.397 g1,3-bis(isocyanatomethyl)-cyclohexane are added thereto. 2 mL anhydrousDMF and 8 drops of dibutyl tin dilaurate are then added. The flask isimmersed in an oil bath at 65° C. and left under stirring for 72 h. Atthe end of the reaction, all the DMF is vacuum distilled at 40° C. and aweight of approximately 8.9 g of compound PU1AT1.b is recovered.Step 3:

To a twin neck oven-dried round-bottomed flask, 0.516 g PU1AT1.b polymerare placed with a magnetic stirrer. The flask is placed in a vacuumovernight. The flask is then placed under a stream of nitrogen and 20 mLanhydrous THF are added. The flask is immersed in an ice bath and 0.2 mLBH₃.Me₂3 are added. Stirring is maintained for 2 h 30, then successivecold additions are made of 0.1 mL distilled water, 00.1 mL 3M NaOH, 0.5mL THF, 0.17 mL absolute ethanol and 0.18 mL 30% H₂O₂. The mixture isbrought to 40° C. for 3 hours under vigorous stirring. At the end of thereaction, it is concentrated then 70 mL CH₂Cl₂ are added to the residueand the precipitate is filtered. The collected filtrate is concentrated,washed and extracted with CH₂Cl₂ several times. Further concentration isperformed and 5 mL methanol with 40 mL ethyl ether are added, followedby filtration and concentration. 415 mg of the compound PU1AT1.c areobtained.Step 4:

To a dried, three neck round-bottomed flask, 0.980 g of2-chloro-1-methylpyridinium iodide are added. The flask is fitted with adropping funnel containing 500 mg dried PU1AT1.C, and with a seconddried single neck round-bottomed flask containing 350 mg1-(3′carboxyphenyl)-3-(2″-hydroxyethyl)-3-methyltriaz(l)ene andconnected via tubing to the three-neck flask. The assembly is placedunder nitrogen, 60 ml anhydrous THF are added to the reaction mediumwith 134 mg methacrylic acid and 165 mg distilled triethylamine.Stirring is carried out for 15 minutes then the content of the secondflask is added by pouring. Stirring is conducted for 3 hours and then 5g triethylamine and 20 mL anhydrous THF are again added to the additionfunnel. The dissolved PU1AT1.c polymer is then added to the reactionmedium and it is left under stirring for 24 h at ambient temperature. Atthe end of the reaction, the reaction medium is concentrated andfiltered. The precipitate is washed in ethyl ether and the filtrate isconcentrated. The obtained crude is dissolved in 40 mL dichloromethaneand the organic phase is washed with 10 times 50 mL water at pH 5-6. Theorganic phases are dried on sodium sulfate and concentrated. The residueis washed several times in pentane and 415 mg PU1AT1 is obtained.

IV. Synthesis of the PH2AT1 Bifunctional Monomer

Step 1: Synthesis of the Precursor1-(3′-carboxyphenyl)-3,3-di(2′-hydroxyethyl)triazene PH2AT1.a.

6.857 g 3-aminobenzoic acid are dissolved in a mixture of 16 mLconcentrated hydrochloric acid (35%) and 35 mL water cooled by an icebath in a twin neck round-bottomed flask. The flask is fitted with anaddition funnel in which an aqueous solution of sodium nitrite has beenplaced (3.45 g in 40 mL) to which is added drop by drop the solution ofaminobenzoic acid while controlling the temperature of the solutionwhich must remain below +2° C. After addition, stirring is maintainedfor 30 minutes. The solution obtained is collected and stored at 0° C.In another twin neck round-bottomed flask, another aqueous solution ofdiethanolamine (10.5 g) is prepared saturated with sodium carbonate.This solution is cooled by an ice bath. Under vigorous stirring, thediazonium salt solution, to which crushed ice is regularly added, isadded drop by drop for 45 minutes and stirring is maintained for afurther 1 h30. After coupling, concentration is conducted in a rotaryevaporator to recover the carboxylate.

The excess sodium carbonate is removed by distillation of the solid in aminimum volume of acetonitrile and filtration. The filtered solution isconcentrated. The sodium carboxylate salt is washed through the additionof 75 mL methanol and 3 mL 35% NaOH aqueous solution. The solutionobtained is stirred for one hour at ambient temperature and thenconcentrated. Finally 45 mL water are added then, under thermalequilibrium with an ethanol bath at −30° C. and stirring, 6.8 mL of asolution containing 3 mL acetic acid and 17 mL water is added. Underconstant stirring a sufficient number of drops of 35% concentratedhydrochloric acid are added gradually until a true precipitate isobtained in a solution whose pH must be reduced to 4-5. Under theseconditions, 150 mL of very cold water are added and filtration isimmediately performed through a glass sinter. After washing in coldwater until filtrates are neutral and removing a maximum amount ofwater, the precipitate is placed in a vacuum oven at 30° C. for a wholeday and 8.92 g of PH2AT1.a product are obtained.

Step 2: Polycondensation of1-(3′carboxyphenyl)-3,3-di(2″-hydroxyethyl)triazene to Obtain PH2AT1.b

To an oven-dried, three-neck round-bottomed flask, passed in a Bunsenburner flame alternately under a vacuum and a stream of nitrogen, areadded 0.5 g dry PH2AT1.a and 0.124 g dry DPTS(4-(N,N-dimethyl-aminopyridinium)tosylate).

Using a syringe, 3 mL anydrous DMF are then added. Under stirring, 0.45mL N,N-diisopropylcarbodiimide are added with a syringe over a totaltime of 4 and a half days.

At the end of the reaction, the urea precipitate is filtered through aglass sinter, then repeated precipitation of the polymer is caused bypouring the solution obtained in minimum volumes of a very cold (10%methanol/90% water) mixture and at least an equivalent volume of crushedice. The filtrate obtained after filtering through a glass sinter issystematically extracted following an identical protocol afterconcentration in a rotary evaporator. On completion, 0.164 g of a darkbrown solid of the PH2AT1.B polymer were recovered after a 2-dayresidence time in a vacuum oven at a temperature of 35° C.

Step 3: Synthesis of an inert shell ofpoly(2,2′bis-hydroxymethylpropionic acid): Obtaining the PH2AT1.cpolymer.

To a three neck round-bottomed flask dried as in the preceding step, 0.3g PH2AT1.b are added together with 12.5 g 2-chloro-1-methylpyridiniumiodide. The flask is connected to a second, dried, single neckround-bottomed flask containing 5.5 g oven-dried 2,2-bis(hydroxymethyl)propionic acid. The medium is placed in an inertatmosphere and immersed in a temperature-controlled bath at 30° C. Usinga syringe, 100 mL distilled triethylamine are added and 5 ml anhydrousDMF. Under constant stirring, in successive solid portions, the2,2-bis(hydroxymethyl)propionic acid is added over a period of 48 h. Atthe end of the reaction, the mixture is vacuum concentrated at 4 mmHgand extracted several times with dichloromethane. A solid is collectedthrough glass sinter and washed several times with aqueous solutions ofpH 6-7 then again with dichloromethane and finally collected and placedin a vacuum oven for 2 days at 35° C. 3.4 g PH2AR1.c are obtained.

Step 4: Methacrylic Functionalization of PH2AT1.c to Obtain PH2AT1:

To an oven-dried three neck round-bottomed flask surmounted by arefrigerant and fitted with an addition funnel, 3.4 g PH2AT1.c are addedwith 15.37 g DMAP. The medium is brought to 50° C., placed undernitrogen and 500 mL anhydrous THF are added using a syringe. To theaddition funnel 13.75 g (0.2 weight % hydroquinone) of methacrylicanhydride are added using a syringe. Finally, 100 mL anhydrous THF areadded to the addition funnel and the addition of methacrylic anhydrideis started drop by drop for 4 hours under vigorous stirring. Whenaddition is completed, the temperature is left to return to 25° C.overnight. The reaction crude is concentrated in a rotary evaporator anddissolved in 300 mL ethyl acetate. This phase is extracted several timeswith volumes of 300 mL water at pH 5. The organic phases are groupedtogether, dried, concentrated and passed through R545 celite withdichloromethane. The organic phases are concentrated, re-dissolved in300 mL ethyl acetate and extracted with volumes of 300 mL water at pH 5.After drying on magnesium sulfate, the organic phases are concentratedand 3.55 g PH2AT1 are obtained.

According to an essential characteristic of the invention, thephotosensitive adhesive composition contains initiation means whoseconstitutents may be most varied in relation to applications and desiredproperties. Several variants of the initiation means can be considered,each one suited to the polymerization process (radical, cationic oranionic) chosen by the formulator. In particular, the initiation meansmay be multiple and in this case the initiation system is amulticomponent initiation system which may give rise to hybrid or dualchain polymerization or crosslinking.

The examples given below do not limit the invention in any way and areintended to enable the formulator to make the best choice amongavailable compounds, in particular in relation to desired use of theadhesive composition of the invention.

In a first variant of initiator means, these are of chemical type.

For radical chain polymerization, examples are limited to thedescription of particular use in the area of dentistry or medicalprostheses. Therefore said initiator means may consist of at least twocompounds reacting together under a redox mechanism to generate freeradicals. A more particular example, routinely used in dental art,consists of mixing an electron aceptor such as benzoyl peroxide ordibenzoyl peroxide with an electron donor such as tertiary arylamine.Another possible example is the oxide of tri-n-butyl borane.

For cationic chain polymerization, the initiator of chemical type may bechosen from among:

-   -   a) Brönsted acids such as perchloric acid,        trifluoromethylsulfonic acid, trifluoroacetic acid, iodohydric        acid,    -   b) Lewis acids, of which the most common are BF₃, AlCl₃, TiCl₄,        SnCl₄, SbCl₅, the Lewis acid optionally being associated with a        weak acid or a cationising agent such as water, a carboxylic        ester, a sulfonic ester, an ether or alkyl halide.

Finally, anionic initiation for an adhesive composition of the inventionmay be used in accordance with the type of polymerizable units chosen,with:

-   -   Lewis bases such as benzylsodium, for example,        phenylisopropylpotassium or the (n-, sec-, tert-) isomers of        butyllithium.    -   bases such as KOH or amines.

However, for the implementation of this polymerization process, personsskilled in the art may be led to taking some precautions, such as theaddition of complexing agents, for example Et₂Zn to protect sensitivefunctional groups.

Also, anionic initiation of chemical type, for an adhesive compositionof the invention, requires special conditions (highly anhydrousconditions, controlled atmosphere for example) which may limit their useto specific applications of the adhesive composition or require thepreparation of a polymerized adhesive of the invention that is appliedto the surfaces of the items to be bonded after polymerization time.

In addition, the adhesive composition of the invention, coming withinthe scope of the first variant of the initiator means, is most often inthe form of “adhesive sub-compositions” optionally comprising additivesolvents, which the formulator must mix before use and before coatingthe surfaces to be bonded to obtain a complete photosensitive adhesivecomposition of the invention able to provide required adhesivenessbetween the parts to be bonded and so as to avoid any prematurepolymerization of the adhesive composition or any long-term storageproblem.

According to a second variant of the initiator means, these may consistof at least one photoinitiator able to initiate the polymerizationmechanism under the action of crosslinking radiation whose wavelength λ1is preferably fairly different to that of uncrosslinking radiation λ2.In this disclosure, λ1 and λ2 designate both single wavelength radiationand ranges of wavelengths centered on the given values. Also, theexpression “crosslinking radiation” denotes electromagnetic radiationcapable of stimulating the generation of free radicals, cations oranions. The expression “uncrosslinking radiation” denoteselectromagnetic radiation capable of causing the hardened adhesive tolose at least part of its integrity.

1—According to a first aspect of this variant of initiation means, theseare photoinitiation means at wavelength λ1.

For the mechanism of radical chain polymerization, this first aspect hasthree versions:

a) In a first version of this aspect, the photoinitiator is of the typeable to generate free radicals via a mechanism of homolyticphotocleavage. The photoinitiators giving rise to such processes belongto various families and the photoinitiators of the invention may bechosen from the categories derived from the following known examples:benzyl dialkylcetal, benzoin ether, α-hydroxy, α-alkyl phenylketone,cyclohexanol benzoyl, oxides of trimethylbenzoyl phosphine and moregenerally bis-acyl-oxides of phosphine, α-amino thioalkylphenylketone,α-amino morpholino-phenylketone, sulfonic esters of α-hydroxymethylbenzoin. There also exist numerous other photocleavable initiatorsin which radical generation occurs subsequent to a series of consecutivehomolytic cleaving processes. The chief examples known to personsskilled in the art are the esters of benzoyl oxime, arylaryslsulfides,peroxides, peroxides containing a chromophor such as benzophenone or anyalkylphenone, disulfides, ketosulfides and azoic compounds such as AIBN(azobisisobutyronitrile) or azobenzoines.

b) In a second version, the photoinitiator is of the type able to createfree radicals through a mechanism of atom snatching. The most frequentclass of these photoinitiators is in the one in which a proton issnatched from a substrate during photoreduction of a triplet nπ* stateof the photoinitiator, the main examples being the derivatives ofbenzophenone, of thioxanthones, of benzyl, of 1,2-diketones such ascamphorquinone, and ketocoumarins. Formulators have at their disposalother photosensitive compounds, onium salts such as the salts oftriarylsulfonium in particular, the salts of alkylarylsulfonium anddiarylhalonium salts, largely described in particular by J. V. Crivello.These compounds are generally given wide used as photoinitiators forcationic polymerization (cf. appropriate paragraph below); however, theinteraction of the triplet state with a proton donor produces freeradicals also enabling initiation of a radical process.

c) The third version concerns the use of photoreducible photoinitiators.The creation of free radicals is consecutive to electron transfer. Thefamilies of compounds behaving in this way are chromophors ofdiarylketone, camphorquinone, ketocoumarin type or aromatic dyes ofxanthene, fluorone, thioxanthone, thiazine, acridine, anthraquinone,cyanine, merocyanine, benzopyrane type. The family of photoreduciblearomatic dyes is likely to be of especial interest to formulators ofdental resins insofar as these photoreducible aromatic compounds areexcitable in wavelengths of the visible range.

Regarding cationic chain polymerization, this first aspect also comes inthree versions:

-   -   a) the photoinitiator is of the type capable of generating a        cationic species, more particularly a HX Brönsted acid under λ1        radiation, by direct photodecomposition optionally in the        presence of a proton donor, optionally by interacting with        polymerizable functions such as epoxide functions for example.        The photoinitiators of the invention may be chosen from among        the onium salts of diarylhalogenium, triarylsylfonium,        dialkylarylsulfonium or dialkyl-phenacylsulfonium type. For        example one of the following compounds may be chosen:        bis[4-(diphenylsulfonio)-phenyl)sulfide bishexafluorophosphate,        triphenylsulfonium tetrafluoroborate, hexafluoroantimonate of        (S-methyl-5-dodecyl-5-phenacyl)sulfonium, hexafluoroantimonate        of (4-n-decyloxyphenyl)phenyl-iodonium, hexafluorophosphate and        tetrakis(pentafluorophenyl)borate of        4-methylphenyl-4-(1-methylethyl-)phenyliodonium.

Conventionally, the counter-anions are chosen from among: BF₄ ⁻, PF₆ ⁻,AsF6⁻, SbF₆ ⁻, RSO₃ ⁻. With a view to increasing the sensitivity in thenear UV or even the visible, structures may preferably be chosen such as4-thiophenoxy triarylsulfonium or the alkylaryl derivatives(9-phenylthioanthracenyl)-10 sulfonium, the bisiodonium salts having forexample covalent oxy, carbonyl, sulfonyl bridges between thediaryliodonium groups.

b) the photoinitiator is of the type capable of generating a Brönstedacid or Lewis acid under □1 radiation. The photoinitiators of theinvention may be chosen from among the diazonium salts in the absence orpresence of hydrogen donors.

c) the photoinitiator is of the type capable of generating a cationicspecies, more particularly a Brönsted acid, under the action of cleavageof a photoinduced covalent bond as is the case for example forderivatives of N-(trifluoromethoxysulfonoxy)phtalimide or of6,7-(trifluorosulfonoxy)coumarin, the sulfonic esters of 2-nitrobenzylderivatives, all generating CF₃SO₃H.

Finally for this first aspect of the photoinduced initiation variant,formulators may have recourse for anionic chain polymerization to thefollowing compounds given as examples:

-   -   halides of trimethylfluorenylammonium and        trimethylbenzydrylammonium;    -   base photogenerating o-nitrobenzyl derivatives, for example        dimethyl        4-(o-nitrophenyl)-2,6-dimethyl-1,4-dihydro-3,5-pyridinedicarboxylate,        derivatives of 2-nitrobenzyl-carbamates, bis(benzophenone        oxime)N,N′-hexamethylene diurethane, N-alkylated nifedepine;    -   O-acyloxime derivatives such as O-phenylacetyl-2-acetonaphthone;    -   quaternary ammonium dithiocarbamates.

2—According to a second aspect of the photoinitiation variant, theinitiation system may be a bi-component system(photoinitiator+co-initiator). Within the present invention, thedescription of this aspect is limited to radical chain polymerization inwhich the two components together produce electron transfer underradiation at wavelength λ1. This is only a particular case of theconcept of bi-component photoinitiation.

One possibility is the combination of an electron acceptorphotosensitive species and an electron donor species. This combinationis generally the one that is most described it enables numerousassociations to be considered. Hence, the electron acceptorphotosensitive species include all the families previously set forth inrespect of photoreducible photoinitiators. Within this strategy, theelectron donor species accelerates radical polymerization over atwo-step process, namely electron transfer followed by proton transfer.Numerous species exist that are able to interact with photoreduciblespecies and compounds may be chiefly cited which include an activatednitrogen atom such as triethanolamine, a tertiary amine or a tertiaryarylamine (N,N-dimethyl-p-toluidine, N,N-diethanol-p-toluidine,N,N-dimethyl-sym(m)xylidine, 3,5-di-tert-butylaniline,N,N-dimethyl-p-ethyl aminobenzoate), the compounds comprising anactivated nitrogen and sulfur atom such as the thiazole derivatives(mercaptobenzothiazole), boron salts (borate) such as tetraphenylboratesor butyltriphenylborates and other salts of tetraalkylammoniumtetraorganylborate type.

Onium salts also hold an important place among bi-component systems whenassociated with boron salts, such as the derivatives oftetraphenylborate and, more preferably, derivatives ofbutyltriphenylborate. On account of strong absorption below a wavelengthof 300 nm and a “tail” of the absorption spectrum in the visible beyonda wavelength of 400 nm, even 430 nm in some cases, these compounds arephotosensitive both in the UV and in the visible range.

3—According to a third aspect of the photoinitiation variant, theinitiation system is a a photoinitiation system comprising, in additionto the photoinitiator proper, at least one species able tophotosensitize the initiator means creating the active centres (freeradicals, cations or anions); this species is called photosensitizer. Inthis case, the interaction between the photosensitizer in one of itsexcited states with the photoinitiator may principally, this descriptionnot being exhaustive, be of two types. Photosensitization operateseither by singlet state-singlet state or triplet state-triplet stateenergy transfer, or by electron transfer photoinduced with thephotosensitizer, or else by both at the same time.

If at least one of the photoinitiators is of the type able to generatefree radicals by homolytic photocleavage mechanism, then a certainnumber of systems exist in the literature which enable this process tobe accelerated. For example an important class of initiators is theperoxides, but the photosensitization mechanisms are most varied. Forexample, benzoyl peroxide, decanoyl peroxide may be photosensitized byenergy transfer from the triplet state of compounds such as anthracene,acetophenone, methoxy- and cyanobenzophenones, associated with theformation of a charge transfer complex. In other cases, electrontransfer is involved as between a thioxanthene for example with3,3′,4,4′-tetra-(t-butylperoxycarbonyl)-1-benzophone. These examples areonly given by way of indication, having regard to the variability ofsensitization mechanisms in relation to operating conditions and thechemical species involved.

Other examples of importance concern the interaction which existsbewteen an α-amino acetophenone (e.g. □-morpholinothiomethylphenylketone) and a photosensitizer of thioxanthone type. Thissystem has the particular feature of being able to function both byenergy transfer from the triplet state of thioxanthone and by electrontransfer.

It is also possible to photosensitize some bi-component systems whichassociate an electron donor and acceptor. In particular, the onium salt,“intermediate” photoreducible sensitizer and electron donor trio is veryefficient in initiating acrylic formulations. In particular thefollowing combinations may be made between:

-   -   an onium salt chosen from among the diaryliodonium salts and        triarylsulfonium salts,    -   an intermediate photosensitive and photoreducible species chosen        from among the categories designated by the following examples:        di(aminoarylketone), ketocoumarin, thioxanthone, xanthene,        fluorone, thiazine, acridine, anthraquinone, cyanine,        merocyanine and benzopyrane, and    -   an electron donor such as the compounds containing an activated        nitrogen atom, for example a tertiary amine and a tertiary        arylamine (N,N-dimethyl-p-toluidine, N,N-diethanol p-toluidine,        N,N-dimethylsym(m)xylidine, 3,5-di-tert-butyl-aniline,        N,N-dimethyl-p-ethyl aminobenzoate), the compounds containing an        activated nitrogen and sulfur atom such as the derivatives of        thiazole (mercaptobenzothiazole), boron salts (borate) such as        tetraphenylborates or butyltriphenylborates, and other salts of        tetraalkylammonium tetraorganylborate type.

The system may be considered differently depending upon whether the“intermediate” species or the onium salt is considered asphotosensitizer.

Also, another series of examples applicable to the scope of theinvention relates to the photosensitization of an onium salt, oftriarylsulfonium or diarylhalogenium salt type used, withoutassociation, with an electron donor of tertiary amine or borate salttype (but optionally associated with a suitable proton donor).Formulators may then optionally choose to photosensitize throughaddition of a photosensitive compound enablng triplet-statetriplet-state energy transfer chosen from the following list: acetone,1-indone, acetophenone, 3-trifluoromethyl-acetophenone, xanthone, or byincorporating a photosensitive compound enabling electron transfer withan onium salt chosen from among: anthracene, pyrene, perylene, aromaticketones such as benzophenone, Michler ketones, xanthones, thioxanthones,derivatives of dimethyl aminobenzylidine, phenanthraquinones, eosine,ketocoumarins, acridines, benzofuranes.

For cationic polymerization reaction, the photoinitiating system inaddition to the photoinitiator chosen from among the onium salts, alsocomprises at least one photosensitizer of this photoinitiator. Theadvantage of this combination is to accelerate photopolymerization andto extend the spectral response of the photoinitiation system.

The photosensitization mechanisms of onium salts are complex and thecomposition of the invention may, as examples, comprise the followingcombinations (examples have already been given for photosensitizationunder radical initiation):

-   -   onium salts and aromatic polycycles such as derivatives of        anthracene, fluorene, pyrene;    -   onium salts and dyes such as xanthene, thioxanthene,        merocyanine, acridone, tetrabenzoporphyrine, flavine, acridine;    -   onium salts and carbazole derivatives;    -   onium salts and metal salts able to achieve electron transfer        towards the iodonium salt;    -   onium salts and ketones such as thioxanthones for example,        derivatives of benzophenone, ketocoumarins, 1,2-diketones such        as camphorquinone, derivatives of anthraquinone;    -   onium salts and generators of radicals such as benzoin ether for        example, dialkoxyacetophenone or phosphine oxide benzoyl.

4—According to a final particularly privileged aspect of this variant ofinitiation means for the polymerization reaction, the latter areinitiation means of photochemical type, whose composition may include atleast one photoinitiator, at least one co-initiator, at least onephotosensitizer or, more generally, any adequate combination between thedifferent potential components of a photoinitiation system, such asdescribed under 1-, 2- and 3- above, with a view to increasing itsefficacy and/or to modify its absorption range of electromagneticradiation.

According to a third variant of the initiation means for thepolymerization reaction of the adhesive composition of the invention,the initiation means are of thermal type. This variant can be given moreparticular consideration for radical polymerization. A great number ofthermal radical initiators exist in the literature. Non-limitativeexamples are: AIBN, benzoyl peroxide, tert-butyl hydroperoxide,tert-butyl peracetate, tert-butyl peroxide, dicumyl peroxide and others.

Finally, a last variant of polymerization initiation means is a globalvariant integrating all possible, compatible combinations of initiationmeans such as described previously in the first, second and thirdvariants. This choice may prove to be a shrewd choice since it canprovide improved crosslinking of the adhesive and improvedphysicochemical properties. For example, in dental use, apost-polymerization or post-crosslinking agent may be added to acamphorquinone/tertiary arylamine system, the agent possibly being aperoxide such as dibenzoyl peroxide.

To conclude, it will be noted that in the case of a radicalpolymerization mechanism, the vinyl units forming a donor/acceptor pairpresented above are also radical initiation means.

It will be easily understood that formulators may evidently combineseveral types of bifunctional monomers within one same adhesivecomposition of the invention. Particular attention must be given to thewavelength ranges used. Preference is given to the combination ofbifunctional compounds for which wavelength □2 is identical oroverlapping, and whose photoinitiation wavelengths λ1 are fairlydifferent. However, if the photoinitiator used is very efficient and ifthe concentration of photocleavable units is reasonable, considerationmay be given to the overlapping of absorption ranges λ1 and λ2 if thekinetics of active centre generation by the photoinitiator are muchgreater than the kinetics of bifunctional monomer cleavage, which maytranslate as exposure of the material to lower radiation powers, longerlamp distances etc.

Also, monomers may advantageously be combined which polymerize bydifferent processes, taking care that initiation means for thepolymerization reactions are adequated, so as to form interpenetratingnetworks whose advantages are well known: mechanical synergy, improvedresistance over time and increased snatch resistance.

According to an essential characteristic of the invention, thebifunctional monomers must be present in sufficient quantity in theadhesive composition to obtain loss of integrity and adhesiveness of thehardened composition when it is subjected to uncrosslinking radiation.The minimum quantity is 0.5 weight % of the adhesive composition. Theoptimum quantity of bifunctional monomers to obtain the desired loss ofintegrity of the adhesive is related to several parameters, inparticular the photoreactivity of the photocleavable units, thestructure of the bifunctional monomers, the presence and type ofco-monomers or reactive diluents of the adhesive composition. Generally,the quantities are low if the glass transition temperature of thepolymerized adhesive is less than −30° C. for example, and greater ifthe adhesive compositions are highly charged and/or crosslinked.

Evidently, the adhesive composition of the invention may, in addition tothe bifunctional monomers and initiation means, comprise otherconstituents in particular:

-   -   co-monomers polymerizable by a chain polymerization mechanism,        which may act as reactive diluent, and which may be of same type        as the polymerizable units described previously or they may be        any polymerizable unit conventionally used in adhesive resins;    -   a component carrying at least one thiol function, such as those        cited in U.S. Pat. No. 4,663,416 and U.S. Pat. No. 4,780,486;    -   a polyalkenoic acid such as a copolymer of itaconic and        polyacrylic acid, for example,    -   various fillers which may be organic and/or inorganic, in        particular of silica type (optionally silanized) or of ionomer        glass type such as CaF₂, YF₃, AlF₃, more generally suitably        formulated fluoroaluminosilicate glasses;    -   additives such as those usually used in adhesive compositions        and intended to improve some of their physicochemical        properties, such as solvents (water and organic), pigments,        stabilizers, surfactants, plastifiers, etc.

The multiple possible combinations enable best adjustment of viscosityor fluidity of the adhesive composition, in relation to needs connectedwith end use.

Also, it will be understood that the photosensitive adhesive compositionof the invention is applicable to numerous areas having regard to themultiplicity of possible variants. Special consideration is given todental use for cementing elements to the surface of teeth and/or to filldental cavities.

In respect of the positioning of bands for orthodontic correction, theenamel surface of the teeth is prepared by cleaning and optional mordantpreparation using appropriate products. Then, the adhesive compositionof the invention is applied to the prepared area, the bands are placedin position on the adhesive layer which is then hardened byphotopolymerization for example.

Once correction has been completed, the bands are removed by subjectingthe hardened adhesive to uncrosslinking radiation which disrupts theintegrity of the adhesive and makes it possible to separate the bandsfrom the enamel surface of the teeth without causing any mechanicaldamage to this surface.

The adhesive composition may also be used to ensure temporary closing ofa tooth root canal. For this purpose, after conventional root dressingand preparation, the root canal is filled as far as the apex of thetooth with the adhesive of the invention, then a master-cone is insertedinside the root canal of similar length to the root canal. Thismaster-cone is made of material able to conveycrosslinking/uncrosslinking radiation on its outer end towards theinside of the tooth and to diffuse the same towards the canal wall sothat the root sealing adhesive of the invention can be easily removedwithout causing any damage to the integrity of the root canal.

Several examples of photosensitive adhesive compositions in theUV/visible ranges especially intended for dental application are givenbelow.

Evidently, these examples can be transposed to any other industrialsector in which there is a need for temporary adhesiveness, which can becontrollably released and without the use of complex or hazardousequipment by operators. Special consideration is given to all systemsintended to be subsequently recycled, in which the use of an adhesive ofthe invention enables very easy separation of various bonded elements.

Attention must be given however to the efficacy of the cleavagegenerated by uncrosslinking actinic radiation. In this respect it ispossible to increase the proportion of bifunctional monomers. It is alsopossible to contemplate inserting an optic fibre guide inside the bondedjoint with an external access point which may be used to transmitactinic radiation for polymerization and then cleavage to the entirebonded joint. The other alternative consists of using the photosensitiveadhesive of the invention to bond parts whose material or form areadvantageously designed so that they can allow the passing of at leastthe uncrosslinking wavelength λ2, initiation of polymerization possiblybeing chemical.

Finally, the photosensitive adhesive composition of the invention mayevidently comprise bifunctional compounds that are polymerizable andcleavable in wavelength ranges other than those described, and theexamples of composition given below are evidently only particularillustrations which do not in any way restrict the areas of applicationof the photosensitive adhesive composition of the invention.

EXAMPLES OF PHOTOPOLYMERIZABLE FORMULATIONS OF THE PHOTOSENSITIVEADHESIVE COMPOSITION OF THE INVENTION FOR USE IN DENTISTRY

In the following examples, the mechanical properties of adhesivecompositions were tested whose formulation comprises one or morebifunctional monomers.

The different constituents are dispersed in a minimum volume of ethylether. The mixtures are homogenized by ultrasound under magneticstirring and are then concentrated and placed in a vacuum of 4 mmHg forone hour.

The formulations are arranged in a Teflon mould whose base is a glassslide to give bars of size (14 mm×4 mm×1 mm) after polymerization.Polymerizations (λ1) are conducted using an Efos Lite Mercury-Xenon 50Wlamp positioned at a distance of 0.5 cm from the surface of the sampleand, unless otherwise indicated, using an interference filter allowingradiation of between 426 and 480 nm to pass. The sample is irradiated150 s on each side, unless otherwise specified. Successive irradiations(λ2) are performed using the same lamp positioned at 0.2 cm from thesample with an interference filter allowing radiation of between 320 and480 nm to pass. The sample is irradiated 600 s on each side.

The mechanical properties were evaluated with DMA (Dynamic MechanicalAnalysis) following a 3 bend-point mode after polymerization at λ1 andafter degradation at λ2.

For all tested compositions, fissuring was observed in the structure ofthe sample after degradation.

The weight percentages of the constituents marked with an asterisk (*)are given in relation to the total weight of the monomers (allpolymerizable organic compounds).

Example 1 Cationically Polymerizable Composition

The composition given below may be used in particular for cementingapplications, but can be used for other dentistry operations.

The sample is polymerized by 300 s radiation on each side. Componentweight % 3,4-epoxycyclohexylmethyl 3,4- 22 epoxycyclohexanecarboxylateCyclohexene oxide 4 NTS 14 Bis-(4-dodecylphenyl)iodinium 1.5hexafluoroantimonate* Camphorquinone* 0.75 Ethyl4-dimethylaminobenzoate* 0.4 Cem-bridge type filler (Pierre Rollanddental 60 products)

The elastic modulus measured at 25° C. after photopolymerization is 2.20GPa and 0.95 GPa after degradation.

Example 2 Anionically Polymerizable Composition

The composition given below may be given particular application forcementing but can be used for other operations in the area of dentistry.

In this particular case, wavelength λ1 lies between 400 and 480 nm andpolymerization times are 300 s on each side. Component weight %3,4-epoxycyclohexylmethyl-3,4- 4 epoxycyclohexanecarboxylateBis-(3,4-epoxycyclohexylmethyl)adipate 20 AT9 16 N-methylnifedipine* 1.5Cem-bridge type filler (Pierre Rolland dental 60 products)

The elastic modulus measured at 25° C. after photopolymerization is 2.40GPa and 1.15 GPa after degradation.

Example 3 Radically Polymerizable Composition

The composition given below preferably has clinical applications forcavity filling and cementing, in particular for root canal work,fissures and anchoring. Component weight % Bisphenol A dimethacrylate9.2 Hydroxyethyl methacrylate 6.2 Butyl methacrylate 3.6Triethyleneglycol dimethacrylate 12.1 PU1AT1 30.7(4-n-decyloxyphenyl)phenyl-iodinium SbF₆ ⁻* 1 Camphorquinone/4-ethyldimethylaminobenzoate* 0.5 Benzoyl peroxide* 0.3 Revolution Formula 2filler (Kerr) 38.2 Hydroquinone methyl ether* 0.2

The modulus of elasticity measured at 25° C. after photo-polymerizationis 2.90 GPa and 0.70 GPa after degradation.

Example 4 Radically Polymerizable Composition

The composition given below has preferable clinical application forrestorative dental operations using composites. Component weight %Urethane Dimethylacrylate 3.4 Butyl Methacrylate 2.3 Polyethylene glycoldimethacrylate Mn˜875 2.3 Triethyleneglycol dimethacrylate 4.5 PH2AT17.15 AT3 3.6 (4-n-decyloxyphenyl)phenyl-iodinium SbF₆ ⁻ 1Camphorquinone/4-ethyl dimethylaminobenzoate* 0.5 Benzoyl Peroxide* 0.3Kappalux M filler (Pierre Rolland dental products) 76.75 Hydroquinonemethyl ether* 0.2

The elastic modulus measured at 25° C. after photo-polymerization is8.20 GPa and 1.30 GPa after degradation.

Example 5 Radically Polymerizable Composition

The composition given below has preferable clinical application fororthodontic cementing, in particular the cementing of metallic and/orceramic parts to a tooth Component weight % Urethane dimethacrylate 5.8Hydroxyethyl methacrylate 7.26 Triethyleneglycol dimethacrylate 5.8 NT410.20 Irg 819* 1 Glycerol Phosphate Dimethacrylate 1 Cem-bridge filler(Pierre Rolland dental products) 69.94 Hydroquinone methyl ether* 0.2

The elastic modulus measured at 25° C. after photo-polymerization is4.65 GPa and 0.80 GPa after degradation.

1-38. (canceled)
 39. Photosensitive adhesive composition ofpolymerizable resin type whose hardening is obtained by polymerizationand/or cross-linking characterized in that said composition contains:means for initiating at least one chain polymerization reaction in orderto ensure hardening of said composition, and a sufficient quantity of atleast one bifunctional monomer including firstly a photocleavable centrecomprising at least one photocleavable unit, and secondly at least twopolymerizable units linked by covalent skeletons to said photocleavablecentre and positioned either side of the cleavage site or sites of saidphotocleavable centre so that said hardened composition loses itsintegrity and adhesiveness under the action of uncrosslinking radiationwhich causes cleavage of the photocleavable units.
 40. Adhesivecomposition as in claim 39 characterized in that the initiation meansfor chain polymerization reaction(s) are photoinitiating meansconsisting of at least one photoinitiator able to initiate thepolymerization reaction mechanism under the action of crosslinkingradiation whose wavelength λ1 is different to wavelength λ2 foruncrosslinking radiation.
 41. Adhesive composition as in claim 39,characterized in that the photocleavable unit(s) of the photocleavablecentre are aryl-diazos units defined by formula I:

in which: Ar designates an aromatic system, monocylic or polycylic,carbocyclic or heterocyclic, including atoms such as S or N inparticular, each cycle preferably comprising 5 or 6 atoms, and is theremainder of an aromatic amine, X designates an atom chosen from among:C, O, P, S; Ri is one or more of the following groups: hydrogeno,halogeno, alkyl linear or branched, saturated or unsaturated, optionallysubstituted, aryl aromatic or heteoaromatic, substituted orunsubstituted, alcoxy such as methoxy for example or ethoxy, aryloxy,alkylthio, arylthio, benzyl, halogeno, hydroxy, hydroxyalkyl, thiol,alkyloxycarbonyl, aryloxycarbonyl, cyano, carbonyl, formyl, amino,carboxylic and sulfonic ester, carboxylic sulfonic and phosphoric amide,carboxylic sulfonic and phosphoric acid, sulfonate, phosphonate,—OCONR′R″ group, —OCO₂R′, —OSO₂R′, —OPOOR′OR″, —R′NHCOOR″, —R′OCO₂R″,—NR′R″ (in which R′ and R″ represent an alkyl group, carbocyclic orheterocyclic group, aliphatic, unsaturated, (hetero-)aromatic group, allsubstituted or unsubstituted, imine whether substituted or not, nitro,—N═N—R′, -Rp-Si-(ORq)₃ group (in which Rp is a hydrocarbon chain,preferably a linear alkyl chain comprising at least 3 C atoms, and Rqdenotes a hydrogen atom, a hydroxy group, C₁-C₆ alcoxy chain or—(Si(ORq) group), vinyl group, acrylic group, alcoxycarbonyl group, anaryltriazene group, Rj designates one or more substituents dependingupon the valency of the atom designated by X, the same or different andchosen from among: an alkyl chain linear or branched, saturated orunsaturated, acyclic or cyclic, optionally substituted; an aromatic orheteroaromatic group including, for example in a preferred chain of 5 or6 atoms, at least one nitrogen or sulfur atom, monocyclic or polycyclic;an alcoxy, aryloxy chain or benzyl group.
 42. Adhesive composition as inclaim 41, characterized in that X designates P and in that thephotocleavable unit(s) of the photocleavable centre arearylazophosphonate units Ar—N═N—PO(OR′) (OR″), in which R′ and R″ areindependently chosen from among: an alkyl chain linear or branched,substituted or unsubstituted, (un)saturated, (a)cyclic, carbocyclic orheterocyclic, a (hetero)-aromatic radical, more particularly ahydroxyethyl chain, 1,4- or 1,3-dimethyl cyclohexyl,1,4-dimethylparaphenyl, a methyl, ethy, propyl, isopropyl, hydroxyethyl,cyanoethyl, acryloxyethyl group, ether of alkyl(C₁-C₆)glycidyl oralkyl(C₁-C₆)vinyl, cyclochexyl epoxy.
 43. Adhesive composition as inclaim 41, characterized in that X designates S and in that thephotocleavable unit(s) of the photocleavable centre arearylazosulfonates Ar—N═N-So(OR′)(OR″), R′ and R″ being independentlychosen from among: an alkyl chain linear or branched, substituted orunsubstituted, (un)saturated, (a)cyclic, carbocyclic or heterocyclic, a(hetero)-aromatic radical, more particularly a hydroxyethyl chain, 1,4-or 1,3-dimethyl cyclohexyl, 1,4-dimethylparaphenyl, a methyl, ethy,propyl, isopropyl, hydroxyethyl, cyanoethyl, acryloxyethyl group, etherof alkyl(C₁-C₆)glycidyl or alkyl(C₁-C₆)vinyl, cyclochexyl epoxy. 44.Adhesive composition as in claim 41, characterized in that X designatesS and in that the photocleavable unit(s) of the photocleavable centreare arylazosulfone units Ar—N═N—SO₂R′, R′ being independently chosenfrom among: an alkyl chain linear or branched, substituted orunsubstituted, (un)saturated, (a)cyclic, carbocyclic or heterocyclic, a(hetero)-aromatic radical, more particularly a hydroxyethyl chain, 1,4-or 1,3-dimethyl cyclohexyl, 1,4-dimethylparaphenyl, a methyl, ethy,propyl, isopropyl, hydroxyethyl, cyanoethyl, acryloxyethyl group, etherof alkyl(C₁-C₆)glycidyl or alkyl(C₁-C₆)vinyl, cyclochexyl epoxy. 45.Adhesive composition as in claim 41, characterized in that X designatesS and in that the photocleavable unit(s) of the photocleavable centreare arylazosulfide units AR-N═N—S—R′, R′ being independently chosen fromamong: an alkyl chain linear or branched, substituted or unsubstituted,(un)saturated, (a)cyclic, carbocyclic or heterocyclic, a(hetero)-aromatic radical, more particularly a hydroxyethyl chain, 1,4-or 1,3-dimethyl cyclohexyl, 1,4-dimethylparaphenyl, a methyl, ethy,propyl, isopropyl, hydroxyethyl, cyanoethyl, acryloxyethyl group, etherof alkyl(C₁-C₆)glycidyl or alkyl(C₁-C₆)vinyl, cyclochexyl epoxy. 46.Adhesive composition as in claim 39 characterized in that thephotocleavable unit(s) of the photocleavable centre are aryl-triazeneunits defined by formula II:

in which: Ar designates an aromatic system monocyclic or polycyclic,carbocyclic or heterocyclic, inlcuding in particular atoms such as S orN, each cycle preferably having 5 or 6 atoms and is the remainder of anaromatic amine, Ri is chosen from among the following groups: alkyllinear or branched, saturated or unsaturated, optionally substituted,aryl aromatic or heteoaromatic, substituted or unsubstituted, alcoxysuch as methoxy for example or ethoxy, aryloxy, alkylthio, arylthio,benzyl, halogeno, hydroxy, hydroxyalkyl, thiol, alkyloxycarbonyl,aryloxycarbonyl, cyano, carbonyl, formyl, amino, carboxylic and sulfonicester, carboxylic sulfonic and phosphoric amide, carboxylic sulfonic andphosphoric acid, sulfonate, phosphonate, —OCONR′R″ group or —OCO₂R′,—OSO₂R′, —OPOOR′OR″, —R′NHCOOR″, —R′OCO₂R″, —NR′R″ (in which R′ and R″represent an alkyl group, a carbocyclic or heterocyclic group,aliphatic, unsaturated, a (hetero-)aromatic group, all substituted orunsubstituted, imine substituted or unsubstituted, nitro, —N═N—R′,-Rp-Si-(ORq)₃ group (Rp and Rq as defined in claim 3), a vinyl group,acrylic group, alcoxycarbonyl group, an aryltriazene group, R1 and R2are chosen independently from one another, a —N═N—R′ group, —NR′—N═N—R″group, OH group, NR′R″ group, (R′ and R″ have the previously givendenotations), an alkyl group such as methyl, ethyl, propyl, isopropyl,butyl, tert-butyl, an alcoxy group substituted or not, a benzyl group, a(hetero)-aromatic group, all substituted or not by substituents of Ritype, a hydroxyethyl, cyanoethyl, aminoethyl, acryloxyethyl,halogenoethyl group.
 47. Adhesive composition as in claim 39,characterized in that the photocleavable unit(s) of the photocleavablecentre are 2-nitrobenzyl units having the formula 1:

in which: Ar designates an aromatic or heteroaromatic radical (includingan atom such as N or S for example) monocyclic or polycyclic andcarrying at least one Rk substituent Rk designates an auxochromic orbathochromic substituent which may be chosen from the followingexamples: hydrogen, halogen, alkyl chain, aliphatic acyclic saturated orunsaturated, linear or branched, a cyclic, aliphatic, unsaturated,aromatic or heteroaromatic radical preferably having 5 to 14 atoms,preferably 5 to 6, these chains and radicals possibly being substituted,interrupted or terminated by a heteroatom such as B, N, O, Si, P, S or ahalogen, a nitro group, cyano group, an alcoxy, aryloxy, alkylthio,arylthio, benzyl, aylalkyl, hydroxy, thiol, alkyloxycarbonyl,aryloxycarbonyl, carbonyl, formyl, amino radical, carboxylic ester,amide, sulfonic ester, sulfonic amide, carboxylic acid, sulfonic acid,sulfonate, phosphonate, a —OCONR′R″ group, —OCO₂R′, —OSO₂R′, —OPOOR′R″,—R′NHCOOR″, R′OCO₂R″, NR′R″ (R′ and R″ are an alkyl, aryl group, acarbocyclic or heterocyclic group), imine substituted or unsubstituted,diazo —N═N—R′, -Rp-Si(ORq)₃ group (Rp and Rq as defined in claim 3),alkylglycidyl ether, alkylvinyl ether, cyclohexyl epoxy. R_(m1)/_(Rm2)are independently chosen from among: a hydrogen, an alkyl, alcenyl,alcynyl, alkylaryl chain, all substituted or unsubstituted, preferablyC1-C6, a carbocyclic or heterocyclic chain saturated or unsaturated,aromatic or heteroaromatic, substituted or unsubstituted, preferablyhaving 5 to 6 atoms, an alcoxy, aryloxy, alkylthio, arylthio chain, analkyloxocarbonyl group, —NR′COR″ group, —OCOR′ group, —OCOOR′ group,—OCONR′R″ group, NR′COOR″ group, —OPOR′R″R′″ group, —OSO₂R′ group,—OPOOR′R″ group, —NR′R″, —COOR′group, —CONR′R″, SOOR′, —COR′ group (R′,R″ and R′″ have the previously indicated denotations for R′ and R″), animine group substituted or unsubstituted, a hydroxy, thiol group, acarboxylic acid or derivative of carboxylic acid, a halogen, a nitrile,an alkyl(C1-C6)glycidyl ether group, alkyl(C1-C6)vinyl ether group,cyclohexyl epoxy, -Rp-Si-(ORq)₃ group (Rp and Rq as previously defined).48. Adhesive composition as in claim 39, characterized in that thephotocleavable unit(s) of the photocleavable centre are 2-nitrobenzylunits defined by formula 2:

in which: Ar is as defined in claim 9, R_(m4) is defined asR_(m1)/R_(m2) in claim 9, R_(m3) is chosen from among a hydrogen, analkyl, alcenyl, alcynyl, alkylaryl chain, all substituted orunsubstituted, interrupted by a heteroatom such as N, O, P, Si, S,preferably C₁-C₆, a carbocyclic or heterocyclic chain, saturated orunsaturated, aromatic or heteroaromatic, substituted or unsubstituted,preferably having 5 to 14 atoms, preferably 5 to 6, an alkyloxocarbonylgroup, NCOOR′ group, —POR′R″R′″ group, —SO₂R′ group, —POOR′OR″ group,—COOR′ group, —CONR′R″, COR′ group (R′, R″ and R′″ having the previouslyindicated denotations for R′ and R″ in claim 9), an alkyl(C₁-C₆)glycidylether group, alkyl(C1-C6)vinyl ether group, cyclohexyl epoxy,-Rp-Si(ORq)₃ group (Rp and Rq as defined in claim 3).
 49. Composition asin claim 39, characterized in that the polymerizable units of thebifunctional monomer are radically polymerizable and are vinyl groupsdefined by formula IV:

in which R3, R4, R5 are substituents able to activate together thedouble vinyl bond vis-à-vis radical addition chain reactions, at leastone of said substituents being a hydrocarbon chain advantageously aC1-C6 alkyl chain.
 50. Composition as in claim 49, characterized in thatat least one of the substituents R3, R4 and R5 is chosen from among thegroups: aryl, carbonyloxyalkyl, carbonyloxyaryl, carboxy (—COOH),alcoxy-carbonyl (—O₂CR), carbamoyl (—CONR₂) and cyano.
 51. Compositionas in claim 40, characterized in that said composition comprises atleast two types of complementary vinyl units, capable of creating acharge transfer complex (electron donor/acceptor pair) itself able toinitiate a radical reaction under the action of crosslinking radiationof wavelength λ1, or at least one type of acceptor vinyl unit able tocreate a charge transfer with another complementary species. 52.Composition as in claim 51, characterized in that the donor vinyl unitis chosen from among the elements: styrene, vinyl acetate, vinyl ether,exomethylene dioxolane in particular 4-methylene-2-phenyl-1,3-dioxolane,alkyl methacrylate, vinyl pyrrolidone, vinyl carbazole, vinylnaphthalene, while the vinyl unit of acceptor type is chosen from amongthe elements: maleic anhydride, acrylonitrile, diethyl fumarate,fumaronitrile, maleimides.
 53. Composition as in claim 38, characterizedin that the polymerizable units of the bifunctional monomer arecationically polymerizable and are oxirane groups defined by formula V:

in which at least one of the substituents R6, R7, R8, R9 is ahydrocarbon chain and are chosen from among a hydrogen atom, halogenatom, an alkyl, alcoxy, alkylthio chain linear or branched, saturated orunsaturated, acyclic or cyclic, preferably C1-C6, optionallysubstituted, optionally interrupted by a heteroatom, an aromatic orheteroaromatic aryl group, an aryloxy or arylthio group preferablyhaving 5 to 6 atoms, a benzyl group, imine group, amino NR′R″,SiR′R″R′″, alkyl(C1-C6)oxycarbonyl, aryl(C1-C6)oxycarbonyl, amide,carboxylic and sulfonic ester, sulfonate, phosphonate, a carbonyl group,cyano, —OCONR′R″ group, —OCO₂R′ group, —OSO₂R′ group, —OPOOR′OR″ group,—R′NHCOOR″, R′OCO₂R″ in which R, R′, R″ represent an alkyl group(preferably C₁-C₆) substituted or unsubstituted, aryl (preferably having5 to 6 atoms), carbocyclic or heterocyclic group, aliphatic, unsaturatedor aromatic, substituted or unsubstituted.
 54. Composition as in claim53, characterized in that, for reasons of steric hindrance, two of thesubstituents R6, R7, R8, R9 are a hydrogen atom.
 55. Composition as inclaim 39, characterized in that the polymerizable units of thebifunctional monomer are cationically polymerizable and are vinyl ethersdefined by formula VI:

in which: R10 and R11 are the same or different and designate a hydrogenatom or advantageously a linear or branched C₁-C₆ alkyl chain,substituted or not, saturated or unsaturated, acyclic or cyclic,optionally interrupted by a heteroatom such as O, N, Si, P for example,an aromatic or heteroaromatic aryl group (preferably having 5 to 6atoms), an alcoxy chain (preferably C1-C6), alkylthio chain (preferablyC₁-C₆), arylthio (preferably having 5 to 6 atoms). R12 advantageouslydesignates a linear or branched C₁-C₆ alkyl chain, substituted or not,saturated or unsaturated, acyclic or cyclic, optionally interrupted by aheteroatom such as O, N, S, Si, P for example, an aromatic orheteroaromatic aryl group (preferably having 5 to 6 atoms). 56.Composition as in claim 39, characterized in that the bifunctionalmonomer is of oligomer or prepolymer size and has a comb branchstructure, consisting of a principal linear polymer chain of which eachof the comb branches contains at least one photocleavable unitpositioned on the side of the principal chain and at least onepolymerizable unit positioned at the free end of the branch. 57.Composition as in claim 56, characterized in that the comb branchescontain a photocleavable unit and a polymerizable unit.
 58. Compositionas in claim 39, characterized in that the bifunctional monomer is ofoligomer or prepolymer size and has a hyperbranched structure. 59.Composition as in claim 58, characterized in that the bifunctionalmonomer with hyperbranched structure is synthesized from a precursormonomer of AB₂ or AB₃ type, in particular under a polycondensation orpolyaddition mechanism.
 60. Adhesive composition as in claim 58,characterized in that the hyperbranched structure has a core comprisingphotocleavable units, and a peripheral shell consisting of inert unitsfrom a photochemical viewpoint.
 61. Adhesive composition as in claim 58,characterized in that the hyperbranched structure has a core consistingof photochemically inert units, and a peripheral shell containingphotocleavable units.
 62. Adhesive composition as in claim 49,characterized in that the bifunctional monomer is1,5-bis[4′-(methacryloylmethyl)phenylazomethyl-phosphonate]-diethyleneglycol.
 63. Adhesive composition as in claim 53, characterized in thatthe bifunctional monomer is: 1,5-bis[4′-methyl glycidylether)phenylazomethylphosphonate]-diethylene glycol.
 64. Adhesivecomposition as in claim 49, characterized in that the bifunctionalmonomer is chosen from:1,2-Bis[1-(4″-methacryloylmethyl-)phenyl-3-methyl]triaz(1)ene-ethane;1,2-Bis[1-(4′-(methacryloylethyl)aminocarbonyloxymethyl)phenyl-3-methyl-]triaz(1)ene-ethane;1-(4′-methacryloylmethyl-)phenyl-3-(2″-methacryloylethyl-)-3-methyl-triaz(1)ene;1-(4′-(methacryloylethyl)aminocarbonyloxymethyl)phenyl-3-((methacryloylethyl)aminocarbonyloxyethyl)-3-methyl-triaz(1)ene;1-(4′-methacryloylmethyl-)phenyl-3,3-di(2″-methacryloylethyl)-triaz(1)ene;1-(4′-(methacryloylethyl)aminocarbonyloxymethyl)phenyl-3,3-di(((methacryloylethyl)aminocarbonyloxyethyl)-triaz(1)ene; 1-(3′-methacryloylethylcarboxyphenyl)-3-di(2″-methacryloylethyl)triaz(1)ene;1,2-Bis[1-(3″-methacryloylethylcarboxyphenyl)-3-methyl]triaz(1)ene-ethane;2-methacryloylmethyl-5-(3′-(2′methacryloylethyl)-3′-methyl)triaz(1)ene-thiophene.65. Adhesive composition as in claim 53, characterized in that thebifunctional monomer is chosen from among: 1-(3′-ethyl glycidyl ethercarboxyphenyl)-3-(ethyl glycidyl ether)-3-methyl-triaz(1)ene,1-(3′-ethyl glycidyl ether carboxy-6′-methylphenyl)-3-(ethyl glycidylether)-3-methyl-triaz(1)ene; 1-(4′methyl glycidyl ether)-3-(ethylglycidyl ether)-3-methyl-triaz(1)ene.
 66. Adhesive composition as inclaim 49, characterized in that the bifunctional monomer is chosen from:2-Methyl-acrylic acid5-methoxy-4-[2-(2-methyl-acryloyloxy)-ethoxy]-2-nitro-benzyl ester;2-Methyl-acrylic acid1-(5-methoxy-4-[2-methyl-acryloyloxy)-ethoxy]-2-nitro-phenyl)-ethylester; 2-Methyl-acrylic acid4,5-bis-[2-(2-methylacryloyloxy)-ethoxy]-2-nitro-benzyl ester;2-Methyl-acrylic acid2-(5-methoxy-4-(2[2-(2-methyl-acryloyloxy)-ethoxycarbonyloxy]-ethoxy)-2-nitro-benzloxycarbonyloxy)-ethylester.
 67. Adhesive composition as in claim 53, characterized in thatthe bifunctional monomer is chosen from among:2-[2′nitro-4′,5′-di(oxymethyloxirane)]benzyloxymethyl oxirane;(2-Methoxy-5-nitro-4-oxiranylmethoxymethyl-phenoxy)-acetic acidoxiranylmethyl ester.
 68. Adhesive composition as in claim 56,characterized in that the bifunctional monomer is chosen from:Poly[(14-(2′-aminoacylethyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co(7-(2′-4′-aminoacylethyl)-1,4-dioxa-5-oxo-6-aza-heptane)],Poly[(14-(4′-aminoacylhexyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(4′-aminoacylhexyl)-1,4-dioxa-5-oxo-6-aza-heptane)],Poly[(14-(4′(4″-aminoacylphenyl)methylphenyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(4′-(41″-amminoacylphenyl)methylphenyl)-1,4-dioxa-5-oxo-6-aza-heptane)],Poly[(14-(4′-(4″-aminoacylcyclohexyl)methylcyclohexyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(4′-(4″-aminoacylcyclohexyl)methylcyclohexyl)-1,4-dioxa-5-oxo-6-aza-heptane)],Poly((14-(4′-methylaminoacylcyclohexyl)-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(4′-methylaminoacylcyclohexyl)-1,4-dioxa-5-oxo-aza-heptane)),Poly[(14-(4′-aminoacylbutyl-6-(hydroxymethyl)-1,4,8,11-tetraoxa-12-oxo-13-aza-tetradecane)-co-(7-(4′-aminoacylbutyl)-1,4-dioxa-5-oxo-6-aza-heptane)]in which all these polymers are esterified on the hydroxy group atposition 6 of the copolymer chain by groups of type:-oxycarbonyl-3-[3′-(2″-(methacrylate)ethyl))-3′-methyl-triazene]phenyloxycarbonyl-ethyloxy-(1-methoxy-3-(methacrylatemethyl)-4-nitro)phenyl.69. Adhesive composition as in claim 60, characterized in that thebifunctional monomer is chosen from among:Poly(1-(3′-carboxyphenyl)-3-,3-di(2″hydroxyethyl)triazene),Poly(1-(3′-carboxy-6′-methylphenyl)-3-,3-di(21″-hydroxyethyl)triazene)Poly(1-(4′-carboxyphenyl)-3-,3-di(2″-hydroxyethyl)triazene),Poly(1-(3′,5′-dicarboxyphenyl)-3-(2″-hydroxyethyl)-3-methyl triazene),Poly(1-(3′-carboxyphenyl)-3-,3-di(2′-hydroxyethyl)triazene-co-2,2-bis(hydroxymethyl)propionicacid), Poly(1-(3′-carboxy-6′-methylphenyl)-3-,3-di(2″-hydroxyethyl)triazene-co-2,2-bis(hydroxymethyl)propionic acid),Poly(1-(4′carboxyphenyl)-3-,3-di(2″hydroxyethyl)triazene-co-2,2-bis(hydroxymethyl)propionicacid), ω-functionalized by methacrylate ends with methacrylic acid andits derivatives, such as 2-hydroxyethylmethacrylate, glycidylmethacrylate or 2-isocyanatoethyl methacrylate for example, or oxiraneends of glycidyl type by reaction with an epihalohydrine for example.70. Adhesive composition as in claim 61, characterized in that thebifunctional monomer is chosen from among:Poly(2,2-bis(hydroxymethyl)propionic acid -co-1-(3′-carboxyphenyl)-3-,3-di(2″-hydroxyethyl)triazene),Poly(2,2-bis(hydroxymethyl)propionic acid-co-(3′-carboxy-6′-methyhenyl)-3-,3-di(2″-hydroxyethyl)triazene),ω-functionalized by methacrylate ends with methacrylic acid and itsderivatives, such as 2-hydroxyethylmethacrylate, glycidyl methacrylateor 2-isocyanatoethyl methacrylate for example, or by oxirane ends ofglycidyl type by reaction with an epihalohydrine for example.Poly(2,2-bis(hydroxymethyl)propionic acid), ω-functionalized by anoxycarbonyl-3-[3′-(2″-(meth acrylate)ethyl))triazene]phenyl group or an-oxycarbonyl-ethyl oxy-(1-methoxy-3-(methacrylatemethyl)-4-nitro)phenylgroup.
 71. Adhesive composition as in claim 40, characterized in thatthe photinitiation means for radical or cationic polymerization reactioncomprise at least one species able to cause their photo-sensitization.72. Adhesive composition as in claim 40, characterized in that thephotoinitiation means for radical polymerization reaction also comprisea co-initiator.
 73. Adhesive composition as in claim 72, characterizedin that the photoinitiation means for radical polymerization reactionconsist of the following photoinitiator/co-initiator pair:camphorquinone/tertiary amine.
 74. Adhesive composition as in claim 40,characterized in that the photoinitiation means for radicalpolymerization reaction consist of a photoinitiator which is a bis-acylof phosphine oxide.
 75. Adhesive composition as in claim 39,characterized in that the initiation means of the chain polymerizationreaction(s) are of chemical type.
 76. Bifunctional monomer includingfirstly a photocleavable centre comprising at least one photocleavableunit, and secondly at least two polymerizable units linked by covalentskeletons to said photocleavable centre and positioned either side ofthe cleavage site or sites of said photocleavable centre, characterizedin that it is of oligomer or prepolymer size and has a comb branchedstructure consisting of a principal linear polymer chain of which eachof the comb branches contain at least one photocleavable unit positionedon the side of the principal chain and at least one polymerizable unitpositioned on the free end of the branch, the photocleavable units beingchosen from the aryldiazos defined in claim 41 by formula I. 77.Bifunctional monomer including firstly a photocleavable centrecomprising at least one photocleavable unit and secondly at least twopolymerizable units linked by covalent skeletons to said photocleavablecentre and positioned either side of the cleavage site or sites of saidphotocleavable centre, characterized in that it is of oligomer orprepolymer size and has a hyperbranched structure obtained bypolycondensation or polyaddition of precursor monomers of AB₂ or AB₃type.
 78. Bifunctional monomer as in claim 77, characterized in that thehyperbranched structure has a core consisting of photocleavable unitsand a peripheral shell consisting of inert units from a photochemicalviewpoint.
 79. Bifunctional monomer as in claim 77, characterized inthat the hyperbranched structure has a core consisting ofphotochemically inert units, and a peripheral shell comprisingphotocleavable units.
 80. Method for preparing a bifunctional monomerincluding firstly a photocleavable centre comprising at least onephotocleavable unit chosen from among the aryltriazenes defined in claim46 by formula II, and secondly at least two polymerizable units linkedby covalent skeletons to said photocleavable centre and positionedeither side of the cleavage site or sites of said photocleavable centre,said method successively comprising a synthesis step of thephotocleavable centre, a structural arrangement step of thephotocleavable centre, and an association step associating thepolymerizable units with the photocleavable centre, characterized inthat the synthesis step of an ayltriazene photocleavable unit consistsof: conducting diazotation in inert organic medium in the presence of aLewis acid of type BF₃ or PF₅ or SbF₅ and of an organic nitrite, thenconducting diazoic coupling by adding a compound comprising at least oneprimary or secondary amino group, in a dissociating organic medium inthe presence of a mineral compound of sodium carbonate, potassiumcarbonate or sodium hydrogenocarbonate type.
 81. Method for preparing abifunctional monomer including firstly a photocleavable centrecomprising at least one photocleavable unit, and secondly at least twopolymerizable units linked by covalent skeletons to said photocleavablecentre and positioned either side of the cleavage site or sites of saidphotocleavable centre, said method successively comprises a synthesisstep of the photocleavable centre, a structural arrangement step of thephotocleavable centre, and an association step associating thepolymerizable units with the photocleavable centre, characterized inthat it comprises a creation step to create polymerizable units of vinyltype on the photocleavable centre, consisting of the creation ofacryloyl functions by nucleophilic substitution on an acryloyl carbon.82. Method for preparing a bifunctional monomer including firstly aphotocleavable centre comprising at least one photocleavable unit, andsecondly at least two polymerizable units linked by covalent skeletonsto said photocleavable centre and positioned either side of the cleavagesite or sites of said photocleavable centre, said method successivelycomprises a synthesis step of the photocleavable centre, a structuralarrangement step of the photocleavable centre, and an association stepassociating the polymerizable units with the photocleavable centre,characterized in that it comprises a grafting step to graft vinyl typepolymerizable units onto the photocleavable centre, consisting ofgrafting the vinyl function included in a molecule comprising at leastone reactive function (F1) onto the chemical skeleton of thephotocleavable centre, and also comprising at least one other reactivefunction (F2), by causing these two functions to react via anucleophilic substitution mechanism on an acyl type carbon.
 83. Methodas in claim 82, characterized in that one of the two reactive functions(F1,F2) is an OR group or an —OOCR group.
 84. Method for preparing abifunctional monomer including firstly a photocleavable centrecomprising at least one photocleavable unit, and secondly at least twopolymerizable units linked by ccovalent skeletons to said photocleavablecentre and positioned either side of the cleavage site or sites of saidphotocleavable centre, said method successively comprises a synthesisstep of the photocleavable centre, a structural arrangement step of thephotocleavable centre, and an association step associating thepolymerizable units with the photocleavable centre, characterized inthat it comprises a grafting step to graft vinyl type polymerizableunits onto the photocleavable centre, consisting of grafting the vinylfunction included in a molecule comprising at least one reactivefunction (F1) onto the chemical skeleton of the photocleavable core, andalso comprising at least one other reactive function (F2), by causingthese two functions (F1, F2) to react to form a carbamate bond. 85.Method for preparing a bifunctional monomer including firstly aphotocleavable centre comprising at least one photocleavable unit, andsecondly at least two polymerizable units linked by ccovalent skeletonsto said photocleavable centre and positioned either side of the cleavagesite or sites of said photocleavable centre, said method successivelycomprises a synthesis step of the photocleavable centre, a structuralarrangement step of the photocleavable centre, and an association stepassociating the polymerizable units with the photocleavable centre,characterized in that it comprises a grafting step to graft vinyl typepolymerizable units onto the photocleavable centre, consisting ofgrafting the vinyl function included in a molecule comprising at leastone reactive function (F1) onto the chemical skeleton of thephotocleavable centre, and also comprising at least one other reactivefunction (F2), by causing these two functions (F1, F2) to react to forma β-hydroxyester bond through attack by a carboxylate anion on anoxirane under conditions of nucleophilic catalysis, more particularly inthe presence of catalysts carrying tertiary amino or quaternary ammoniumgroups.
 86. Method for preparing a bifunctional monomer of oligomer orprepolymer size as in claim 77, characterized in that function A being acarboxylic acid function and function B an alcohol or amino function,the polycondensation or polyaddition reaction of the precursor monomersof AB₂ or AB₃ type is conducted in the presence of dehydrating agents.87. Method as in claim 86 characterized in that the dehydrating agentsare chosen from among: 1-methyl-2-chloropyridinium iodide,dicyclohexylcarbodiimide, N,N′-diisopropylcarbodiimide,N,N′-carbonyldiimidazole, 1,1′carbonylbis(3-methylimidazolium)triflate,di-2-pyridyl carbonate, 1-hydroxybenzotriazole, an acylation agent ofPyridine/Tosyl Chloride type or SOCl₂/DMF.
 88. Method for preparing abifunctional monomer of oligomer or prepolymer size as in claim 77,characterized in that function A being an ester function and function Ban alcohol or amino function, the transesterification reaction of theprecursor monomers of AB₂ or AB₃ type is preferably conducted in thepresence of catalysts such as titanates, organic tin oxides and esters,using basic catalysis in the presence of non-ionic bases providing softoperating conditions such as amines, amidines, guanidines,triamino(imino)-phosphoranes.
 89. Use of the photosensitive adhesivecomposition as in claim 39 for various clinical applications in the areaof dentistry, in particular to bond elements to the surface of teethand/or to seal tooth cavities.
 90. Bifunctional monomer includingfirstly a photocleavable centre comprising at least one photocleavableunit, and secondly at least two polymerizable units linked by covalentskeletons to said photocleavable centre and positioned either side ofthe cleavage site or sites of said photocleavable centre, characterizedin that it is of oligomer or prepolymer size and has a comb branchedstructure consisting of a principal linear polymer chain of which eachof the comb branches contain at least one photocleavable unit positionedon the side of the principal chain and at least one polymerizable unitpositioned on the free end of the branch, the photocleavable units beingchosen from the 2-nitrobenzyls defined in claim 47 by formula
 1. 91.Bifunctional monomer including firstly a photocleavable centrecomprising at least one photocleavable unit, and secondly at least twopolymerizable units linked by covalent skeletons to said photocleavablecentre and positioned either side of the cleavage site or sites of saidphotocleavable centre, characterized in that it is of oligomer orprepolymer size and has a comb branched structure consisting of aprincipal linear polymer chain of which each of the comb branchescontain at least one photocleavable unit positioned on the side of theprincipal chain and at least one polymerizable unit positioned on thefree end of the branch, the photocleavable units being chosen from the2-nitrobenzyls defined in claim 48 by formula 2.